I've ordered new FSP couplings and I'm waiting for them to install them. In the mean time, I put on a coupling half of the inferior design. So far I've done eight or so loads without any issues. Looks like it was just the inferior couplings that were failing.
I guess on the bright side, I've performed a decent overhaul of the machine replacing the clutch, pump and agitator assembly, overhauling and cleaning the brake and transmission. When I get the FSP couplings installed I shouldn't have to worry about this machine for several more years. Doubt I'll get another fifteen years out of it, but anything is possible.
Thank you all for the helpful hints and suggestions. And special thanks to Eric (bairbank56) for a dose of reality by pointing out the now obvious issue with knock off replacement parts.