yeh it does bounce around alot but it has for a while now but was told it was due to the bigger tires... but im going to guess this was a lie lol
Sounds like Storm99 was (he often is, if you read properly) right ...
Alternator, small lead un-plugged and touching or covered in ****e, plus maybe alternator bearings/windings going west. Happened to me, swapped out for a new 100amp alt and all was well again ..
Makes the rev-counter bounce about, battery stops being charged or gets over-charged (which is worse), all sorts of electrical gremlins pointing at all sorts of ****e which 'mysteriously' clears up when alternator fixed/replaced.