FT-767GX - 70cms module taking off

[voltz]

Hi all, I haven't posted on this forum for a long time. Hope someone has had some experience with this.

Had a problem with a FT767GX, 70cms keeps going crazy during SSB transmission with PO meter showing full scale intermittently and loss of output. This is an unusual one, traced it down to the 70cms module (the rig is a HF, 6, 2 and 70cms bands). All other bands are fine. Rig is essentially in working order.

When i remove the module case lids, the problem seems to clear up but as soon as the lid gets even just close to the unit, its taking off at around 442Mhz with full carrier. Basically going wild. Placing the lids on firmly with screws makes it unstable, sometimes its ok sometimes its taking off. But i have not been able to locate any faulty component or any bad earthing etc. All earth points have been cleaned / re-tinned. Tried supplementing the earthing to lids with copper foil, with no effect. So my conclusion is - is this a design fault?. The PA stages have too much gain and its self oscillating when the lid gets close to the box. Unfortunately, that means having to mod the thing, to stop feedback. But the question is, has anyone out there also experienced this? Nothing showing on google. Result is a time consuming job - thankfully its my own radio not a customers!

Things I have already tried:
Ferrite rods place inside the 'cavity' areas to de-tune any resonance - no effect
Ferrite beads placed on internal power lines - no effect
Extra screening soldered in sensitive areas on PCB (early PA stages including TX mixer) - helped reduce the effect but problem still persists.
Googled too may times - no mention of this anywhere.

Thanks.