weird ignition issue after installing tach

ok, put my Sun supertach 2 back into the Duster. Wired everything up, then the car wouldn't start. Turned over fine but not firing. I went out and tripped the solenoid to check for spark, and this time it fired right up. Then I realized (after it had warmed up for a couple minutes, so was more agreeable) that it was starting as I released the key, but not until then.
Not a ballast resistor issue ( i have one of those self-contained electronic distributors that don't use the BR or an exterior control module), but here's what happened. I unhooked the ground from the tach to see if it solved the problem (and it did).
But the tach still works-then it dawned on me that because I had screwed the tach to the column, it already had it's ground. Could the second ground somehow cause this issue? Wouldn't think so, but can't figure out why it happened otherwise.
The car starts normally with the tach's ground wire unhooked...nothing is shorting out anywhere, I double-checked all of that...odd.