[Kenwood] Ts940 setpoints

[solo22@sbcglobal.net]

Good day, gentleman.  After a 6 mos absents from ham radio, I have found
the setpoints off on *both* of my 940's.  Using the WWV, upper/lower SSB
buttons and the adjustment on the left side of the radio, I reset both
tones the same.  Now I notice after some time passes, the radio has
drifted enough to change set point function again, requiring reseting,
on *both* 940's.  Casual ragchews or DX chasing are uneffected by this,
it's just group chatting get me complaints about being a little off freq
at times.  My question is this....it this normal parameters for '940's?
Both of mine act about the same. (One's a 10mil, the other a 20mil.)  Is
this why Kenwood offered the SO-1, to help the problem,
or.....???        Any help or comments would be appreciated,  Thank You,
Ron, K6ZL