Imtermittent idle

When at an idle the truck varied between high rpm and idle rpm it does this none stop, even after it is warm. I just had problems with the charging system and ended up replacing the alternator. Could a sensor or relay or something be working only intermitently? It all started when I started it after school and drove a short distance shut it off for a few minutes and tried to start it. I had to jump start it. Headed for home got maybe a mile and it started running rough and wouldn't idle. The battery light and brake light are on. Found the wire from the battery to the fuse box was nearly broken in half, repaired that with new butt connector, cleaned up the battery and terminals, Brought the battery home, charged it, replaced it into the truck and drove home. Nearly 4 miles the truck started running rough. Continued home another 4 miles. Temp here is about 10-20 degrees F. Cahrged battery again drove 1 block and it died. Next day jumped it and drove it home ran fine. Replaced the alternator seemed to be fine until it was warm, then it started the varying high and low rpm thing. Our nearest shop to have the computer codes read is 50 miles away. Any info in this matter is greatly appreciated. Info on how to change and where to find particular sensors etc. Is appreciated also.