[TenTec] Omni VI freq instability, 30m only

[k7gt@charter.net]

I have recently received on-the-air reports of a slight
frequency shift, 30m only, with my option 3 OmniVI. No
reports on other bands. As I only recently have had an
antenna worthy of being called such on 30m, my activity
level there is fairly high now. It may have 'always' been
there. 

Any thoughts as to what might cause this? Bad mixing osc
crystal, out of adjustment? A trip to Sevierville is in order?

Allan K7GT