This machine began to exhibit slippage of the tub belt at the motor pulley, and would intermittently fail to spin. Seems to agitate normally. I replaced the tub seal and bearing, which seemed to help at first, but now the original problem has returned. I have replaced both belts, and cleaned belt residue from motor pulley. The machine fills and drains normally. It will occasionally go through a normal spin cycle, but fails (with belt screeching and burning belt smell) more often than not. I have discovered today that the machine will spin if I "help" it by nudging the transmission by hand. I'd appreciate your recommendation on what to try next.

The most helpful checklist you sent pointed me to belt tension, which turned out to be the problem. I had checked this previously when installing new belts, but upon rechecking, the tub belt was indeed loose, causing slippage. Once again, the obvious eludes me! Thanks ever so much for your expert help.