Tach Voltage/Major Ground Issue?

Problem: I have the stock 165 tach hooked up to my megasquirt, but it's acting flaky. Can someone tell me what the voltage range is from the stock ignitor, going to the tach? I want to make sure my little transistor circuit is working correctly.

Symptoms: Tach works at idle, but above 3000rpm, tach drops to zero. Engine runs fine. When headlights are on, tach drops to zero. When turn signals are on, tach bounces.

Hypothesis: Major ground problem. Do you agree?

Unsuccessful solution: I have the battery relocated to the trunk, so I ran an ~8 gauge ground wire up to the valve-cover, and have the tach circuit ground T'd into this line. It seems to have made no difference...

Next step: I'm going to install a number of 10gauge ground wires from the engine to the chassis and ground the MS directly to the engine...think this will work?

UPDATE:

I added a number of 8AWG ground wires, which join together and go to the EMS and back to the battery, which is in the trunk...however, it doesn't seem to help.

Should I ground the tach directly to the engine/ems, as well?

This car apparently had some gremlins before I got it, and hacked up the wiring to remove redundant wires, once the ems was installed.

Does anyone have any experience with weird tach problems? Maybe I need a tach adapter circuit, like MSD/Autometer sells?