Well, that's a new one to me... but you're likely on the right track with grounding. Suzukis are well known for doing weird things when grounds are bad. Can't say I'd know why only in 5th, though there might be a switch for EGR etc to tell the ECU it's in 5th. Start by cleaning the existing grounds to the block & body. Adding more can't hurt. G/l & let us know what you find, I'm curious!