You really can't tell what is going on with the AC system until you have measured both the low side and high side at the same time. You may need to take it to a shop.
The doors that direct the air through the system and out the vents are VACUUM OPERATED. At least on my '95 Jeep, the vacuum fittings were all a mess, and didn't get any cold air from the AC for a while because the vacuum connector that supplies the vacuum to the doors had come disconnected, which resulted in the door moving to a position that blocked any air coming from the system. Might want to check your vacuum connections.
http://home.howstuffworks.com/ac1.htm
Check the link, it explains the basics of an AC system. Remember Junior High Physics, expanding gas absorbs heat energy and makes things around it cooler, as well, a phase change, like liquid evaporating to a vapor does the same. Basically your refrigerant, in liquid form, is allowed into the evaporator (a radiator) where it evaporates and expands, making the evaporator get cold. It expands and moves down the line to the pump, that compresses and pumps it through the system. When you compress a gas it gets hotter, so the pump moves the compressed refrigerant to a condenser (another radiator) that cools the hot compressed refrigerant to the point that it condenses into a liquid, then it moves from the condenser to the evaporator to start the process all over again. The liquid inlet to the evaporator has some sort of metering valve, usually an expansion valve (which is on Jeeps) or an orifice. There is usually a big cylinder in the system that acts like an accumulator for the liquid/gas, plus a filter and drier (a bag of desiccant). The system is under such pressure and heat, that any air or moisture in the system will form acids that will eat up the system, that is why it needs to be vacuumed down before servicing, to remove any air or moisture.
Basically, the pump moves the refrigerant thru the system, where pressure builds up big in the system up to the evaporator, where there is a little orifice that high pressure liquid is sprayed out to expand at low pressure, then in the evaporator all the way up to the suction side of the pump, the pressure stays real low and cool.
The system sitting still, will have the pressure even out to about 50-75 PSI in most systems. When you start running the pump, the high side will go up to 150-225 PSI, depending on the load on the system. The low side will go up and down around 18-28 PSI, usually with the compressor cycling off once or twice a minute to keep the low side pressure from getting to low. If the low side pressure gets to low, the evaporator will go below freezing, and all the condensation will freeze on the evaporator and air won't be able to flow through it.
If your low side pressure is jumping up to 40 psi while the compressor is OFF, then drawing down to 25 psi while the compressor is on, that would actually be normal. Even if the refrigerant level is low, the system will get pressure like this on the low side. The high side pressure will be low, like 80 PSI and the compressor will cycle on and off a lot if your low on refrigerant. If there is a clog in the system, or there is too much refrigerant in the system, the high side pressure will shoot up to 300 PSI and the high pressure switch will cycle the compressor on and off. See, how you really can't tell unless your looking at the pressure on both the hi and low side at the same time.
It sounds like, but you really haven't given us enough information to tell, that your low on refrigerant, and your letting that gauge on the servicing bottle scare you out of adding more refrigerant, because it jumps up higher than you expect.
You have to have the motor running, with the AC on the highest setting. Hook up your servicing bottle, and start adding refrigerant. While adding refrigerant, it will bump up the pressure in the low side of the system, as well, when the compressor cycles off, the pressure will bump up also. Keep adding refrigerant, I've seen the low side pressure jump up to 60 PSI while adding the refrigerant, just keep adding it. 2 signs that you have enough in the system, the compressor will stop cycling on and off, it should only cycle on and off for a few seconds once or twice a minute, and it will cycle off when the low side pressure drops down below 25 PSI, its actually about 18PSI on my Jeep. As well, watch the low side line coming out the evaporator (it will be the larger, cool line coming out of the firewall) just before you reach the proper amount of refrigerant, that low side line will get cool enough, for long enough that water out of the air will condense on the line and the line should be covered in sweat. Once the line is covered in sweat, add another once or two, then stop adding refrigerant. Watch your gauge, it should stay around 18-28 PSI, with the compressor constantly running, if it gets down to 18-24 PSI, the compressor should cycle off and the pressure jump again, once it jumps again, over 24 PSI, the compressor should turn on again. The compressor should NOT go off for more than a few seconds, 2-10 seconds; and it should only cycle off once or twice a minute. Try that and see if the AC works better.
or the wifes grand cherokee.