[UK-CONTEST] A Weird Dressler D 200 S Linear Amp Problem

[Will M0GYZ]

Hi All

Got a weird one for you.

I have Dressler D200S VHF Linear Amp which was given to me for contesting.
There has always been a strange problem which i originally thought was a 
sticky relay
However after a while of looking at it. Its as if the amp wont Transmit 
without it being on a really flat surface which caused problems for me 
at the recent 144/432mhz contest
I can't figure out why it would require such a dead flat surface before 
the needle moves and im putting power out.

It was working ok apart from this at home as i was putting out 120w with 
5w drive provided I found the right spot for it but when i took it out 
to the contest it just didn't want to Transmit at all and when it did 
the power just decreased over 3-10 seconds and I couldn't get 120w out 
of it max.

My thoughts are thats it might be a dry joint but the guy who had it 
before me didn't ever report it had a problem but unforuntly he died a 
couple of years ago.
So it only Transmits when its in a spot it likes after moving around my 
windowsill

So any thoughts  ?

Thanks
Will M0GYZ