That is typical look from the soldering process. There are jumpers you can install to simulate the bottom board operating as a non directional reverse unit. No top board installed. You can run it that way in forward first. If it runs fine it could isolate to a top board issue. If it stops and goes, no reverse unit to drop in neutral, so periodic hesitation and movement again would indicate issue is in the bottom board. Can be a component over heating and dropping out.
Frankly the easiest method is swapping boards. To many components can cause this. Diodes, rectifier, relay, transistor. If you have another PS-1 engine swap the top board in, chip included. Run it. Doesn't matter if it is steam or not. If problem goes away it is the top board. If it stays, it is bottom board or wiring, so replace bottom board with the original top board and chip test again. G
