Tachometer issues

[AVG7]

I'm hoping to get some help with my 84 944. Several years ago, my tach started to work intermittently, either shutting down completely or getting to 3000 rpm then falling back up to zero. I've changed both sensors and had the DME checked (OK) and resoldered. This was for unrelated problems. When the tach stays at zero, the shift light and the 'mpg' gauge still work. I've checked the body grounds except for one behind the dash. This is hard to get to on my car. I'd like to get some insights on this problem, I've read forums but haven't seen one that sounds just like what I've got. Thanks