1994 Firebird Shudders in Over Drive?

[Jordan D]

My 1994 Firebird has recently began acting up. I live in Connecticut and it has been very cold. When I ease off break at a stop, the car shudders until it goes over 5-10 mph. Then when it reaches 4th gear (overdrive, i have automatic) it shudders violently to the point that it worries me. I have been dealing with this by driving with 3rd gear so i don't reach 4th. It is obvious the effect it has on my gas, but recently I have discovered a temporary solution. I brought it in to a car wash and had the under carriage washed. The car had a VERY apparent boost in performance and ran like it was new. In all gears, the car just felt all around better. I could take it in to 4th gear and it ran fine, same with starting from a stand still. I thought it was fixed for good, but then it sat for 3 hrs in 20degree weather and when i started it, it was back to horrible. I tried a car wash again to test my theory and sure enough it ran beautifully. I drove it for about 4 hours no problem and the next morning to work no problem, after work however it was back to the same old story.

My question is this, what is wrong or possibly wrong? Where should I start looking for problems first? I put Iso-HEET in it to soak up any moisture so far it hasnt changed anything. One auto store told me moisture in the gas, another told me bad connectors going into tranny / torque converter and something about my stall not being connected or getting ice on it?

Any input would be greatly appreciated!
The problem doesn't arise after driving it, but instead they arise if it sits too long and the temperatures are well below freezing.

 

[terry u]

sounds like a loose wire somewhere underneath and when you wash underneath maybe its put back into place until you drive for awhile? just a thought and good luck.