s10 help!!!!!

[trid]

ok so today my truck started to be cool and while i drive the voltage likes to drop to like 10-12 volts then go back up. It has no rhyme nor reason it just does it whenever it wants to. Ive had the alternator and the battery tested today and both are good. We tested the alternator twice and the battery 3 times so they are good. Im good with cars but this is stumping me. Help me please i need to get to nashville
thanks
travis