I have a '88 5.0 HO Lx. It has developed a high idle, right at around 12-1300. but if i leave it idle for a while, at a stop light for instance 5-10 minutes the idle will fall and level at around 7-1000. What can cause this. I've checked my timing it is on the line and my vacuum is also steady and in range. The only thing i can think of is that maybe the spout chip is bad but i don't know how to check it. Also the maybe the tps but i cant remember how to check that either please help.