88 XJ overheating problems

I'm sure this has been covered a bajillion times, I'm starting to really lose patience with this car. It was a gift to my girlfriend so she would have a form of transportation to work, but its been far from a 'reliable source'. I replaced the radiator with one from a newer XJ with a fill neck, I just replaced the water pump about an hour ago because I thought that was the reason it started overheating out of nowhere (it was running extremely cool for about a month) and I took it for a drive and everything seemed fine. I let it sit in park in the driveway to simulate a drive through (which she does often, and the first time iit overheated recently) and it started overheating again. Water was coming out the bottom hose (return?) of the overflow, and when I shut it off, it started bubbling and coming out of the top, like always.

I'm not sure if my overflow is holding pressure, I thought about just getting a hose connector and connecting the overflow hose with the return hose, to force it back through the system instead of bubbling up. Since I converted, would that be a good idea? I'm starting to run out of ideas and patience.

Thanks in advance.

edit: I figured I would also replae the thermostat while I was in there, took off the housing and there wasn't one! Guess that would explain the really cold temperatures.

If it's overheating at idle, then it may be your fan clutch. Grab the fan blade and try and move it front to back. If there is movement, then you need a new fan clutch.

The fan clutch was actually replaced right before we bought it. I just drove it for a long time and it stayed right at the bigger line between 100 and 210 so I think we're good. I went through 2 drive throughs and no problems, I did sit for a while I'm just going to tell her to avoid idling for a long time.

Is there still a way to bypass the overflow in case it does heat up? that would just force the hot coolant back through the system and cool it, correct?