360 runs great one day and then poorly the next

Hopefully someone out there can point me in the right direction. My brother-in-law has a 360 with a fresh rebuilt Thermoquad and having running issues.

New engine compartment wiring, good coil, ballast resistor and electronic ignition module, new distributor and fuel pump. There's no intake leakage and fresh fuel in the tank. Even swapped the TQ with a new Edelbrock with same results.

It didn't matter with air cleaner on or off before. Started it today with air cleaner off and began to ran poorly once again. Installed the air cleaner and it began to clear up. Prior to today, with or without air cleaner installed had no adverse effect. When air cleaner is off the port on the carb is capped.

Shut down and started back up an hour later and would not run like it did before. Runs terrific one day and then the next day runs like crap. Park it for a few days after running good, fire it up a few days later and wouldn't run as it did when parked. There's a gremlin running around!

Any guidance would be appreciated. Thanks, Jack