Pontiac: 1995 Pontiac GP Overheating, hairline crack, losing coolant

Question
Tom, I have a 1995 Pontiac GP with a 3.1 engine. The car has 130,000+ miles. The car had been running a little hot so I flushed the coolant system, replaced water pump, hoses, temp gauge sensor, fan sensor, thermostat and cap. It seemed to help for a few weeks and began to run hot (220F-250F) again and was losing coolant. I finally took it to a qualified mechanic. He pressure tested the system and found no problem. He did however find a leak in the Manifold gasket. After he replaced the gasket, I drove the car for 2 days with no problem. Day 3 I started the car in the morning and was about 3 miles down the road when the temp gauge registered in the red(~255F). After about 45 seconds, the gauge dropped instantly to 185 and ran no hotter than ~205F the rest of the drive (approx. 20 miles). I took it back and let the mechanic check it again. He tested the system again. It happened to him also. He tried a 180F thermostat and tested. He thought he had it licked and drove it for a day. On day 2 the car overheated exactly like it did with me. It immediatly overheated and once the temp came down, it ran at normal temp for the remainder of the drive.  He tested the temp and was getting the same temp at the block which ruled out the car temp guage. It was strange that the top hose on the radiator was not hot. He thinks it maybe a hairline crack in one of the heads. I thought the pressure test would show that and there would be a loss of coolant? I'm at wits end with this. Have you heard of this before? I was thinking something is wrong with the thermostat but we replaced it 3 times since this problem began. I would appreciate any help.  

Answer
NEVER EVER install a LOWER temp thermostat in a fuel injected vehicle as you will drive the computer NUTS. Re-install the CORRECT thermostat.

Your pressure test on the engine at normal operating temp should reveal a loss of pressure due to either a crack in one of the heads or a fault in the head gasket.

There is also a possibly of a coolant passage restriction (radiator).