I had a somewhat similar experience on an 02 3.5L Intrepid Police. I bought it off an online auction. It was specified to have 60-some thousand miles and an overheating problem, but I took a chance on it, winning the bid at $1600. I drove it 13miles home, no problem, but it would slowly build heat just idling in the driveway. I checked the electric fans, the fan relays. I even pulled all the heat exchangers and cleaned out all the dirt and bugs clogging the fins. As a last resort I pulled the t-stat. It was not oem, so someone had replaced it, and put it in backwards. I bought a genuine mopar t-stat, installed it correctly, and I've put almost 100k more miles on it in the last 14yrs with zero problems. In all fairness to the mechanic who installed the t-stat backwards, it is not an easy R&R. It's a rather inaccessible t-stat location, on the side of the block, directly under the exhaust manifold. Unless you dismantle a lot of surrounding accessories, you have to do it sight-unnseen, mostly by feel.
