Sounds like the plow is losing pressure, either one way or another. Don't cushion valves control left to right movement ?.. say when pushing a mound and hitting something, they allow fluid to flow from one side to the other ?
If they're not set properly, or damaged, wouldn't the plow just flop around left to right ?
BTW, mine stays wherever I left it last.
92 Cummins with a 8.5 Western Pro Plow