Idle problem, 88 Carrera

[Decolliber]

After engine has been running for a while, when it goes to idle (say, stopping at traffic light), the engine idles normally for about 10-15 seconds, then drops to a rough 600 rpm idle. I have checked the idle switch (as per Bentley), replaced the idle valve, run through a bottle of Techron, and checked the hoses for visible vacuum leaks. The O2 sensor, spark plugs and wires are near new. Can't think of what else to look for. Is the fact that it idles normally and then drops a clue? There is what sounds like valve noise on the left side, and I am going to adjust them soon, but that is not likely to be them problem, is it?

[john walker's workshop]

try raising the idle speed to 800-850. it's the brass screw in the throttle housing looking right at you. if the yellow plastic cap is still on it, you have to break it off. use a 7mm deep socket and turn counterclockwise. new style temp sensor yet? tighten all the intake nuts. they are usually somewhat loose. the looseness causes air leaks and occasionally an intake gasket blows out causing a larger leak and a dead hole at idle.

[FA-18C]

Had a similar problem with the 87 I had years ago..went through the same things and ended up having the fuel injectors blown out, seemed to solve the problem.