Your explanation has me wondering, what lights are on normally that fail when the tach acts up. be specific exactly what electrical items quit?
There's a number of items that can croak the engine, just a flaky ground at the
intake manifold for example. Making it important to notice as much as possible.

When it quits do you think it's lack of fuel? THEN the RPM falls?
Maybe one of the two fuel relays is crapping out?
Intermittent problems can be difficult to troubleshoot.

You can get some parts at the local salvage yard and save a bundle over dealer prices for items that may or may not be the culprit.

Bill