I have a 2001 ram 2500 5.9 magnum. Its new to me but recently I've noticed an occasional dip in rpms. both when idling and driving. never stalls but does noticeably dip. the other day at idle i though it was going to stall but it dipped and came right back up. any ideas where to start looking. I know the truck hasn't been driven much over the last few years. If that makes a difference.

 

Sounds like a misfire, make sure the spark plug wires and plugs are in good shape. Sometimes it takes awhile for the computer to pick it up and send out a check engine code. Could even be a bad sensor, but still it should throw a code.