T-stat a rollercoaster of emotions

I did a search and found a lot of threads about thermostats etc., but not any quite like this. The temp on my '90 xj is constantly up and down. It seems like the thermo keeps opening and closing. It will get up to 210 or so then drop to about 130 and back and forth constantly. This is the third thermostat in about as many years. The first one did the same thing, the second one was fine until it stuck closed on me and overheated, now this one is another yo-yo. I've also replaced the water pump (about a year and a half ago) and had the system flushed (pretty dirty). All three thermostats have been from Car-Quest, 185 degrees. Any ideas? I'm running out of hair to pull out.

Get your t-stats from the dealer! The OEM ones have a small hole drilled at the 12 o-clock position to allow any air that gets in the system to escape. I've never heard of anyone having a problem with dealer t-stats. Aftermarket ones are a different story......

A few years back, I had LOTS of thermostat problems. Not only with my Cherokee but with other cars too. I just COULD NOT BUY a decent thermostat that would work and last . . . I was replacing them more like every 6-months. I went to NAPA and bought their best heavy duty thermostat and they were supposedly made by Borg-Werner . . . . that seemed to make the difference. Good luck. Don

Also, it may not be the thermostat. That sounds like a bad temp sensor or sender or gauge. May also simply be air in the system. The thermostats at the dealer are not necessary, I have always run NAPA ones and had no problem. You may wish to simply hook up an aftermarket gauge and see how it reads.

There is a bypass built into the block and the thermo housing, that allows a certain amount of coolant to flow around the thermostat, so the temp. at the thermostat is closs to what is actualy in the motor. The bypass is at about horizontal or about 3 oclock, it hooks up to the heater and surge tank lines. The reason for the bleed hole (in the oEM themro) is to let the air out of the very top of the system with the thermostat closed (the air that doesn´t make it through the bypass).
If you use a universal gasket you can partially block the bypass (and heater). In some models the thermostat can go in backwords. With the heat sensor activator pointed to the front, the thermostat operates erratically. The early Jeep for sure, takes a special low profile thermostat, the normal thermostat (CHEV or others) can be crushed a little during installation.
I´ve found squeezing the top radiator hose (with two hands vigioursly and repeatadly), when the motor is at operating temp. helps move the trapped air into the surge tank.
I´d install an OEM thermostat (and gasket) with the little sign stamped into it that says this side out/or front, possible what your getting just isn´t getting the job done. With a good OEM thermostat, you can fairly confidantly and move on to something else in your troubleshooting.
I weak fan clutch (at or near idle), will run your temp up a bit, allow the thermostat to open all the way and then when the RPM´s are increased, run a slight surge of cold coolant through the motor, pretty quick. Remember the temp, sender is at the rear of the block and the thermostat is at the front, takes a moment for the thermo to close, after the cold coolant has arrived. Causes some pretty wild swings on the temp gauge.