'88 Chevy stumbles- seems to be electrical
 

My '88 chevy C1500, 4.3, stumbles the first 5 minutes or so of driving. The problem goes away immediately when the heater fan is turned on. This was a government truck and does not have A/C.

I'm hoping someone with knowledge of the electrical system can at least give me a starting point. There must be a short somewhere but the short does not drain the battery during long periods when the truck is not used.