I have an 87 F250 5.8 liter 4barrel carb
On cold start the truck runs fine for about a minute and a half. Shortly after I kick down the high idle it will start to backfire then stall out shortly after. Turn the truck off wait a few minutes and the problem goes away. If I choose to just drive away after starting cold the problem remains;A minute or two down the road and the backfiring starts, then stall. Wait a minute or 2 and all is fine.

I have been through this truck a hundred times and cannot find a vacuum leak anywhere. I do know the egr valve is functional by putting a vacuum line directly to it. I haven't actually seen it move while connected as it should be. I do have a ported vacuum switch in the control scheme. It has 4 ports but I'm not sure how to check it. I'm sure one is a supply, one to the egr, one to the carb, and one to another control mechanism of some type.

^
||---Black: to some control valvel near the cannisters
||---Red: Vacuum supply
||---Green: to EGR valve
||---White: to carburetor
|==|
|==| Ford # looks something like E5TE-9D473-B2A

This problem has persisted for a couple of years believe it or not. During this time I replaced lots of stuff. Recently I replaced the ignition module. The carb and the rest of the motor is a factory rebuilt. Exhaust is also new.

Any ideas ? Please!