GTP, Because you tach jumps a bit when the cut out happens, I still suspect your HEP. I did see that you changed it but let me offer these thoughts.
Just because you replaced it, doen't mean the new one is fully functional. Stranger things have happened.
Those contacts on the plugs to the HEP switch, you cleaned and insected them, are they tight? The female ends tend to 'open up' and fit loose on the male bullet end of the plugs causing an intermittant issue.
Those wires to the HEP switch often end up hanging or worse under tension by the way the main harness runs low and puts wieght on the relatively small wires to the HEP. I'd do some continuity checking to be sure you don't have an intermittant partial break in the wires either on the switch side or on the harness side.
Did you closely inspect the shutter wheel that the HEP recieves it's reference from? It just has little plastic welds or 'buttons' that hold the wheel to the plastic backing, they can and do fail partially at first causing a wobble, then eventually totally fail causing a walk home malady

A couple very small low profile screws with washers will ensure that they never loosen again.
I've seen the distributor caps installed backwards, there is a small relief in the cap that the flat HEP wires run under. If the cap is backwards, it will crush/break the wires.
The tach recieves it's signal from the HEP which is why I still want you to focus there a bit longer to elimnate any componant of the HEP as a possible culprit. Good that you are observant and caught the little tach jump when the hiccough happens

Alan