if the gauge is going up and down, could be a couple of things, but kind of need more details. Could be your battery cable ends are bad and need to be replaced, they also could be lose. You could also have a loose ground, or if you have underdrive pullies, that will make it go up and down as well.
1994 cobra -
Afr 165s, F303 (too big), full exhaust, tko 600, rebuilt 306 with DSS level 10 block, TFS Street manifold, 3.73s, 31 spline axles, ford racing 31 spline t-lok w/ carbon fiber discs, mm sub-frame connectors, UPR lower control arms, steeda strut tower brace, and lots of other stuff.
1994 mustang gt -
Home ported stock lower manifold, and rebuilt stock heads.