Bicycle Tutor

Looking for help to identify what the cause is for rear derailleur mystery shifts that have plagued me over the past several years. These mystery shifts seem to be random and occur at light to moderate loading, as well as heavy loading.
HARDWARE: LX SL-M570 shifters, XTR RD-M952 rear derailleur, SCRAM PG-990 cassette and SCRAM 991 Chain.
I've tried all the standard efforts, with limited improvements such as changing the chain and cassette and truing the derailleur hanger. The derailleur jockey wheels do not appear to be worn out.
The only thing I have not tried is changing my shift cables and shifters. I do note that derailleur shifts seem to take a lot of force with my thumb, as compared to other bikes I've ridden. I'm not sure if this is generic to the shifter model or a problem with the shifter. Other than the force required, it appears the shifter functions properly.
Before going down a path or changing the shifter, or the shifter and the derailleur, I thought I'd see if anyone had any experience or info that would help find a solution to these mystery shifts.
Thanks in advance... Bill K

Have you removed and cleaned the cable inners?
Also, while you have them out, look for any frayed strands.
With the cable installed, but not clamped to the derailleur, pull the cable end while operating the changer. This should be free with no drag.