Strange coolant temp issue

Hi, I picked up a 99' Grand Am se 160k on the clock powered by the 3.4L about 6 months ago for cheap as a commuter car due to gas prices. After about 2 months of driving, the car overheated one day, but had no coolant loss (and no coolant in the oil). Changed the thermostat and flushed the system and it worked fine for about another month, and then overheated again, once again, no coolant loss (and no oil in the coolant) After feeling around on the hoses, it felt as if the waterpump wasn't pumping coolant, so I replaced that as well (it was fairly cheap considering I'm used to buying parts for fullsized trucks). Also replaced a noisey powersteering pump at that time as well. Another month goes by running and driving fine, and then it overheats AGAIN. With the car running I pop the hood, and both cooling fans are running and they are drawing plenty of air through the radiator, so I know that isn't the issue.

When it overheated the next time I sat on the side of the road for about 5 minutes and turned the car back on and the temp crept back down and was fine (drove another 40 miles home with no problems). Drives fine for a few days and does it again, I repeat what I did the last time, and again it drives fine for the rest of the day. I go back and forth with this for 3-4 weeks then it starts overheating everytime I get in it. And since then I quit driving it. Coolant is clean, it's been flushed a couple times, no oil in coolant and vice-versa(now that I say that there probably will be tomorrow when I go look at it). I'm stumped, there is no coolant in the oil, and the fans work, been flushed 3 times as of now, I've bled the air out according to the book, and everything else is new order as far as I know, but I can't figure out the problem. I have no idea why it will overheat, I can sit there for 3-4 minutes and turn the car back on and it's fine again.

The car has sat for about 1.5 months, and I've decided to sell it at this point, I've reduced the price accordingly to well under what I paid for it, as well as all the other misc. things I've done to the car after I got it. I just need to figure out a more concrete problem to tell potentional buyers. I've found some things that I'll go check out on it later this week thanks to reading this forum for the past 4 hours. But the more help the better and it's all appreciated.

Thanks.

Welcome to AF.

Sounds like air trapped inthe cooling system.
Does your heater blow heat then get cool when it overheats ?
Are you sure bleed all the air out after flushing it ??
Thier is also a procedure in the Tips and Maintenance subforum (http://www.automotiveforums.com/vbulletin/forumdisplay.php?f=2148).

The 3.4 is no stranger to head gasket failures. Take it in and hve the coolant tested for hydrocarbons. If it comes back negative (i would be surprised) you need to monitor the coolant temp with a scan tool to see if it is actually overheating or if the gauge is just reading high.

Ok, apparently I didn't get all the air out the first time around. I rebled the system on Sat. morning and drove the car all weekend and it stayed cool. Never got hot.

Thanks.