My 85 K10, 305, quad carb, no smog stuff....The truck idles up and down when its warmed up...It will idle nice for a few minutes and then it will drop down and then pick right back up and idle fine for a few more minutes...over and over again...it never gets close to shutting off though....
Sound like the carbureator has either dirty idle wells or it has a vaccuum leak around the thorottle shaft. Could also be ignition problems also
Does the truck have air conditioning? if it does the compressor might be cycling on and off. it creates a pretty good pull and will slow the engine down quite a bit. otherwise you could try shootin carb cleaner around carb base on the gasket and on the intake gaskets with the engine idling. If you see bubbles then you have a bad gasket just my $.02 hope this can help
