Washer won't spin during heavy loads

I have a Kenmore 70 series direct drive washing machine that's having problems spinning heavy loads dry.

Problem:

My wife noticed a loud banging during the spin cycle, like it was off balance. That lasted for approx three loads and then stopped by itself. Next we noticed some loads weren't spinning dry but some were. Seems that the heavier the load, the slower the spin. Too heavy and it won't spin at all. The basket drains fine.

During heavy loads when the spinning stops, you can manually turn the basket a few times counter-clockwise which seems to help it regain some traction. But it's still too slow and will quickly stop spinning again.

Attempted solutions:

I pulled the washer apart and inspected the clutch and coupler. The clutch has strong grip and the coupler did too. Both looked and worked good on manual inspection.

What other problems could it be? Basket drive? Gear case? Have you seen this before?

Thanks!