Yeah, JT..Ive only had mine on for 2 days...You say on your 66Grubee it comes and goes? like speedo goes to 0.0? or you mean it reads erratic numbers? All I see to do is make sure the magnet is lined up with transmitter right on mark, no more than 1 mm. I dont think you can get interference out of magneto box intermittently..when I had those wireless ones on they just didnt read right at all, ever. Seems to me that if it works good once, it should keep working as long as magnet misalligns,..... but then what do I know? mine probably'll go out tomorrow!!!