I wouldn't rule out stuck pawl on freewheel or similar freewheel related slippage when under any load. I've had more cases of older, heavily- (or under-) trafficked freewheels causing slippage than worn rings. Easy to test for also - toss on another rear wheel with a known-good freewheel, or just swap on a known-good freewheel.

But sure, could be related to chain or ring wear.