I have an 85 720 that does this too. I don't drive the truck much, but it will remind me that I haven't fixed the problem when the temp drops and I drive it. It will act like it starves for gas, and if you push it it will just about shut down. I have not been able to get under the hood and see if the bowl is full or not while this is happening. I suspect it has something to do with the EVAP system for some reason, but I does not do this all the time and When I get into the truck to do something I generally have more important stuff to do than hunting for the problem. It will usually go away if I stop and let it idle for a couple of minutes and finish warming up. It does not do this if I let it full warm up before I leave with it. I have also known of two other trucks doing the same thing and I don't know if anyone has found the cause yet.