[ICOM] Icom IC-251E Tuning Problem

[Phil S]

Hello!
I've got a bit of a problem with an IC251E 2m transceiver which I'm
hoping that the brains on this reflector might be able to assist with.
As soon as power is applied, the rig starts to drop in frequency on
the display as though someone was tuning it. If I turn the VFO to
increase frequency, it does so, if the upward tuning rate is faster
than the downward rate. If I tune the VFO to decrease frequency, the
drop continues and gets faster if the VFO is turned quickly.
I can lock the frequency by using the lock button but as soon as the
button is deselected the tuning resumes its tumble downwards as though
there was an invisible operator tuning the VFO. 
I can transmit and receive without a problem (whilst locked) and the
frequency display is accurate to the tx/rx frequency.
The rig does not appear to be scanning as the squelch has no impact
upon the symptoms and does not halt the frequency drop when the
squelch opens.
Changing mode makes no difference and neither does changing VFO
selection.
Thinking that an opto detector was at fault, I cleaned around the area
of the VFO etc internally - but to no avail as the fault continues.
Any help would be greatly appreciated. I'd hate for the rig to remain
unused on a shelf until either it, or me, goes SK!
73,
Phil VK4BAA