I have a 2002 Venture with about 56k on it (yes that 56k, not 156k or 256k).

Last year around this time it threw a code for not reaching temperature soon enough - perhaps the thermostat was stuck open. I had the thermostat replaced, along with the temperature sensor, upper/lower hoses, and the (probably first ever) coolant flush.

Fast forward to February or so 2014, and I noticed while getting into work that the heat started to blow cold. Strange but I ignored it.

A couple of weeks ago, the temp gauge started to fluctuate to halfway, blow cold air, then I'd get hot air and the temp would drop. Strange but not awful. Twice on my way home from work (but not on the way to work), the temp gauge reaches red (and today the indicated panel lit red "hot coolant"). Both times I pulled over, let it cool a few minutes, and then I think the thermostat opened and was able to drive home.

My mechanic thinks it's a thermostat, but of course that's expensive. I have read some horror stories of more expensive repairs than a thermostat, so I wanted to get thoughts.

 

Just like you my Venture is an '02 also. However, mine now has 181k miles. It was pretty much worry free the first 8-9 years and 130K miles. Since then it has been a bit of work each year to keep it going. First thing was intake gaskets leaking and loss of coolant. Fixed that and was good for another 20k miles then the headgaskets and overheating problems began. Had to pull the heads and redo the top end. Also changed a couple of thermostats along the way. It is a bit of challenge to completely fill the coolant system and get all the air bubbles out. Lisle makes an excellent spill free funnel with adapter that works really well to ensure a full system. I am guessing that perhaps you have some air pockets that have not been eliminated. Good luck.

 

Good call on the air. I followed a YouTube video on bleeding the air out. Nothing came out by just filling the radiator, but after running it, coolant dripped out of the first, then a few minutes later the second. After that the temperature gauge stayed around 1/4.