Strange problem with tach

[icingdeath88]

Having a weird problem in my 6-speed swapped SVX. The tachometer reads low, sometimes around 2/3rds where it should be and more recently usually about half. (But it still seems to vary, even in the same driving session). Anyone ever experienced something similar?

I'm wondering if there could be a poor connection between the ECU and tachometer that's causing some of the pulses to be dropped? Either that or a problem in the circuitry of either the ECU or the tach causing some of the pulses to either not be sent, or not be received? Or maybe low voltage to some part of the tach causing the response to the pulses not to be as strong as it should?

I don't currently have a spare tach to swap in, so any ideas for how it might be fixed or what to look at would be very much appreciated. I have multimeter/soldering iron/etc, and a non-working tach in a spare gauge cluster that might be usable as parts.

Or, if anyone has a working one for cheap, that would be great, although it probably wouldn't ship very easily if removed from the gauge cluster, though I suppose I could send the rest of the gauge cluster back.

Thanks