Tachometer help?

My 92 w/zf5 has a tach that's reading funky. It seems to read a little low at idle and likely low also at rpms up to 2000 or so, but if I get into it over 2200, it pegs all the way, then comes back when it gets under 2000 or so. I have removed and ohm-tested the sender, it's reading about 2500. The wires were showing some copper where they enter the unit, but I filled around them with some liquid plastic electrical compound. There wasn't any sludge or metal flakes on the sender when removed, but I cleaned it with parts cleaner before reinstalling. I have removed the dash cluster and cleaned the connector/printed circuit connection on that side. I was under the impression that the sender wouldn't be responsible for a overtach reading (high voltage?) like the one I'm seeing. Am I misinformed? Do I need to look at ground issues or can I have a bad diode or something in the gauge? All other gauges/electrics seem to work perfectly.

I don't know how the cluster grounds, and I'm a little leery about going into the dash again. I had to fix a lot of plastic clips with 5-minute epoxy last time...plus it seems that a replacement tach gauge might me hard to come by. I'm just hoping someone can give some insight before I start throwing parts at it. Any ideas?