Interestingly, I swapped out the rear derailleur on the SHAM bike for a 24 year old Shimano 105 RD-1050 6 speed derailleur that I bought at the local community bike co-op for five bucks and the rear shifting is now flawless...Most likely the credit for the improved shifting is due to the RD-1050's stronger spring and the flexibility of the Shimano floating pulley...

My only stumbling block right now is the chain rub with the 8 speed chain and the SRAM Rival front derailleur...I'm going to try and source out an 8 speed Shimano or Sachs front derailleur to see if they'll play nice with the Rival shifter...