[Drake] 20 & 15 Meters fades away on my Drake 2-B

[Mike]

I have a Drake 2B that I have really been enjoying listening to. I haven't
had it very long . It has a little problem though. Most of the time 20 and
15 meters seems dead on it. I know its in the receiver because I can switch
over to another rig and the band is active. Even when the slide switch is
set for a  calibration tone you still can't hear it. Sometimes it will come
back up but only for a few mins and then fade away. I haven't  taken the
case off to look around or spray any contact cleaner on anything.  Was
hoping maybe someone might have a B-2  that has seen this problem or knows
the cause for this. It works great on 80 and 40 meters . Only thing that I
have tried just to see if maybe it might be a crystal was I put the 40 meter
in the 20 meter slot and still nothing .
Does anyone have any input on this or what to check ?
Thanks,
Mike W4MJR