driver side window intermittently will not roll up/down. What could it be? Ford dealer wants to charge $98 just to look at it! (does not include cost to fix it). Any suggestions would be appreciated! :(

hi all, my truck, an 89 s10, runs fine and temp is good when i'm driving. but as soon as i lift my foot off the gas the temp gauge rises a bit. put on the brake to stop and it jumps from 200 (around normal) up to 250. ouch! idling at a light it drops back down to normal after a half minute or so. once it drops, it drops quickly, going down 50 degrees in a matter of seconds. revving the engine in neutral will speed up this process. then driving again and the temp is solid and normal at around 200. water pump looks good, fan clutch is less than 3 years old. thermostat looked good, but didn't test it. hoses are seem ok and there's not coolant loss or drips. did a back flush and also a 10 minute flush. i'm praying it's not the head gasket, but if it is, wouldn't there be water in the oil, or vapor from the exhaust? oh.. and i burped the coolant system, also. at this point i'm scratching my head. any ideas?