[Ottomotion]

I had an experience I will share...

When I converted my Vanagon to Subaru power a few years back, I had a similar event. Driving it and then sudden loss of power...engine died and dash lights came on. It would start right up afterwards sometimes, sometimes it would crank and would not fire...but for some strange reason, it would eventually fire and be fine again. Drove me crazy.
I checked fuel pump, relays, fuses, etc...
I thought for sure I would have to pull the whole shebang out, pull the wire harness and recheck the modified bundle of wires one by one to make sure something wasn't done incorrectly.
I think it is our nature to assume the worst case scenario.
In my Suby Vanagon case, it ended up being a ground nut on the engine that I had failed to tighten down. Snugged it up w/10mm socket and its been great for many years.

Is that whats wrong with yours? Maybe...you did do an engine swap and there are a few ground points involved.

...but maybe not. Point is, don't panic. It could be anything...even something small and cheap.