Originally Posted by Okieron

Sounds like you may have an issue with the charging system. If you haven't had the batteries load tested you have no idea if one is bad or not so start there first.

Thanks alot. Where can I go for that?

Does it do it when the fan comes on automatically? If so I would say check the connections on the alternator, or a wire with a break in it. Loose connections and broken wires cause strange things to happen. Just because a battery has a green light on the check window does not mean that it is good either.

Check your battery box, make sure all connections are tight (loose connections can result in poor charging). Check the fuses, as well.

First test, as said, is go to a truck stop and have them diagnose your truck's electrical system. it's fairly cheap, and will check your batteries, draw against them, charging, etc. My guess is your batteries aren't that good, or you're not getting them charged properly but enough to keep the truck running (mostly) when going down the road.

I would say the FIRST thing to check would be your grounds. Check to see where all your grounds attach to your engine.