Odd tachometer problem - any ideas?

[joeydurango]

My 72 K10 has a factory tach (albeit one that was added later, as it isn't on the SPID). I'm running HEI (so was the PO, but I dropped in a new distributor about 9 months ago).

Most of the time the tach works perfectly, and is actually accurate (I've checked it against my timing light across a wide range). Sometimes, however, I'll start the truck up and the tach will do nothing at all - it will just be dead. More oddly, sometimes it will react to engine RPM, but only up to the highest recent RPM, stay pegged there, and slowly bleed back down over the next several minutes. (Or, it will stay at highway RPM when I turn it off, then drop back if I cycle the key.) Even more oddly, from either dysfunctional state, one of two things will happen: 1) I'll turn the truck off, and the next time I fire it up it will be back to normal, or 2) I'll be driving along and suddenly the tach will just spring into action. It is the strangest thing and is slowly driving me nuts.

I have checked the connections on the back of the tach and at the distributor, and they appear to be fine. I have even checked them while the truck is running with the tach not working (unplug and replug at the distributor, fiddle with the fitting on the gauge), to no avail. The tach just seems to have a mind of its own.

I suppose I don't have a thorough understanding of how it works, beyond receiving impulses from the distributor. For example, I know my speedo gets "bouncy" when it's cold out due to extra friction in the cable; but I'm not certain of potential causes for the tach issue. Anyone seen this before, or have any ideas? Thanks!