If it isn't fixed by the wire jiggle, and odds are it won't, try running a separate ground wire "from" the frame back behind the instrument cluster "to" the back metal plate of the instrument cluster somewhere. Almost any screw location will work.
This has fixed many a persons jumping gauges.....

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John