EGR code with oscillating idle rpm - 93 Silverado with 5.7 automatic. Anyone with an older truck ever have this problem. ? Truck runs great but intermittantly, at idle, the ...

93 Silverado with 5.7 automatic. Anyone with an older truck ever have this problem. ? Truck runs great but intermittantly, at idle, the rpm will oscillate and on occasion will turn on the service engine light. New EGR valve and solinoid. New Oxygen sensor. Problem seems to be accentuated by the addition of a throttle body spacer. The engine has been overhauled and bored .030 over. This problem has existed for years, (before overhaul) but since the engine was freshened up it is more pronounced. Have had 3 separate ECM's. All have had no effect. I changed the first one due to an unrelated problem and found that as soon as I got a good ECM installed the check engine light would come on intermittantly. The 32 code (EGR) is all it ever flags. Had never seen the check engine light before on this vehicle. I bought it new in 93.
Any thoughts?

Thanks,

You have an engine vacuum leak.