2252 Vertical dc drift

Hello. I have a 2252 scope with a fault thats driving me mad. Both channels 1 and 2 exhibit DC drift. ie. input is grounded, so centre the trace with the vertical position knob. Switch to DC coupled and the trace goes up to about 10 mV. As the scope warms up this increases
to about 60mV. Going back to grounded input brings the trace back to the normal preset position. Switch to AC input and the shift is still there only a slightly different value. CH3 and 4 seem OK although they have much reduced sensitivity so hard to see the shift.
I have the service manual but am drawing a blank. Any ideas or am I doing something really stupid?