[Boatanchors] DX 100 losing grid drive on 80 mtrs only

[wa1zuf at juno.com]

Here's my problem:

Transmitter has plenty of grid drive on 160, 40 & 20. (10 is low, 
however it was low before I had an issue on 80) When I first got the 
rig, 80 worked fine, When the drive started to fail it was 
intermittent, dropping down while in QSO I would have to turn up the 
drive control and then is would suddenly return. I have cleaned (using 
de-ox-it) and inspected all the rotary switch's paying close attention 
to the band sw. 

Working into a dummy load, I can get 4+ ma of drive on 3.500 (still 
low as compared to 160 and 40) as  move up the band, the drive drops 
down to 1.5 ma by the time I get to 3.875. (I have also checked it 
with q 3.85 crystal with no improvement noted) 

I had a spare 12by7a, (of unknown condition) when I tried it the drive 
drop further down. 

So what avenues can any of you folks recommend?

Thanks,

W1TAV - Steve