Randall RH150 volume cutting

[Durrant Pickups]

Hi i have a Randal RH150 head and randomly the output cuts in and out this happans on all channels. The only thing that rectifies it is i have to plug my highest output guitar in and strum it really hard (dimebucker pickups). This amp is called "valve dynamic" uses mosfet power and has a single ECC81 valve in it which i have replaced but hasnt fixed the issue. any insight would be appreciated.

[g1]

The most common cause of this problem is discussed here: http://music-electronics-forum.com/t28549/