Frigidaire dishwasher ~25 years old. Little used. Sat completely unused for a few years due to failed building water supply.

Water has been restored and it seems to run OK, but machine often fails to drain. The pump sounds like it's running but nothing comes out. Other times it seems to work fine.

I've checked everything and the only explanation I can come up with is that the pump is failing and sometimes just fails to start when energized.

This is very similar to an issue I read about here from 10 years ago. That problem was apparently never solved. IIRC the OP replaced the pump but the problem persisted.

My drain hose is pretty long. I have it hung in a loop against the back wall before it goes to the sink drain connection. I can blow air through it and there's no sign it's ever had a kink.

I read somewhere about a check valve. Could it be sticking? Where is it located? Is it part of the pump? Could it be part of the drain hose where it connects to the pump?