Bad DME Relay???
 

So, I have an 86 3.2 in my 1974 911. On occassion, the engine completely loses power for a split second as if someone has flicked the key off then immediately back on. Thoughts? Is that a DME relay or likely something else?

Mine did the same for 6 months. I replaced everything and it kept doing it. Gave up and put in MegaSquirt.

I actually had this happen for a while in my 1972 911 with a 3.2.
It was the actual DME (not the relay).
I had the DME replaced (exchanged)
and that resolved the issue.

Since you don't seem to have a spare one, time to get one and find out as you should not drive around without having a proven working one in the glove box.

I have two new ones in the glove box!

Swap them around and see if you problem still persist.

Then in less than 2 minutes you can answer your own question. Plug one in and see if the problem stops.

problems like this may only happen once a week, or even once a month.

start with the cheap stuff or easy stuff.
don't forget the crank sensors.

do the dash lights come on?
wiggle wires around with the engine running.
spray the engine with water with it running

I have a 964 3.6 in my '80SC and had a similar problem to yours. My problem would occur when hitting hard bumps or jolts. I finally found the problem, the DME ECU aluminum case was not adequately grounded
to the car body. My ECU lives under the right seat and I added a discrete
ground wire with a star washer to the floor sheet metal. My not help you,
but fixed my problem.

Check DME not just relay - including chip is in properly - bumps or power loss like that could be loose something or other and an easy fix but tough to track down

Open up your old DME relay and look for broken solder joints on the back of the circuit board. Reflow as needed (or do them all preventatively).

Same thing can happen with the DME itself, but it's a bit more sensitive, so be careful or leave it to someone with some electronics experience.