I think the ELD just measures the loads, I wonder why it got messed up by a shorted fan relay... Is that fan relay in the same fusebox? Maybe it messed up some connections underneath?

Runs fine for 20 seconds then bad... I can't think of anything obvious. I'd probably go around & verify all the basic stuff. You want to rule out as much stuff as you can. Plugs condition & gap, spark timing, plug wires arcing, vacuum leaks around the FITV & TB...

Maybe put a timing light on it, watch to see whether the spark timing does anything strange at that 20-second point?