Too Fast Idle

[grs]

The engine idle is OK til I run the truck a mile or two. Then it idles real fast. Does not happen all the time. Checked all the vac hoses. There is no leak. I turned the engine off, and restarted it. Then it idles OK. I think it is the vac. advance in the distributor hanging up. What do you think? I want a GM dist. Do dealers still them? Thanks
It idles about 100 rpm's faster. It has an electronic dist. in it. It has a rebuilt carb. I took the carb to a real good shop. It only idles fast after a high speed drive. It will idle fasts til I turn it off , and then turn it back on. If I race the engine up in the garage , and then back it off it wont idle fast. If I turn it off, and then back on it will idle at normal speed.