[Elecraft] K2 goes wild on 40m - - need advice

[vidi]

Elecrafters,

I bought a K2 already built up from a friend  while on a trip to the USA.  It is serial number 1008.  It is stable on all bands except on 40m where it breaks into oscillation as soon as I start sending CW, even with the power output set to a very low level.   
I  would appreciate  advice from any K2 owner  who has experience of this problem and could help me rectify it.

73
Vidi - ZS1EL