Tektronix TDS7104 Making SPC Calibration stick?

[CaptDon]

We have a well kept TDS7104 here at work. It is in excellent condition but
has one small problem. At startup there is an annoying amount of D.C.
offset in channel 3. The offset is stable and unchanging even after complete
warm-up. Simply running Signal Path Calibration removes the offset but the
unit does not seem to store the correction factor. If you exit the cal menu and
do a normal power down, the offset will be there again even if you restart the
scope immediately while it is 'still warmed up' so to speak. Is there some secret
handshake to get this thing to store the offset value?? Thanks!!