thermostat?
 

I have a trans am and sometimes when I first start it up, after it has cooled off, I will go to drive it and it will shake really hard back and forth, but when I put it in neutral it stops. It doesn't do this when it is warm. Another thing is that when this is going on the temperature gauge is still not showing any increase. What I dont understand it why it doesnt do it when it is warm. Could it be the 160 thermostat I have?