A problem with my Watson W-25AM power supply

My Watson W-25AM power supply has a serious problem. When I switch it on everything seems fine. The voltage needle reads 13.8v and the lights are on and everything seems fine and dandy (see photo below). The PSU will power my FT-2500M and my FT-897 while both are in receive mode.

However, everything goes awry when I key up on either radio. The voltage needle suddenly drops and when I release the microphone key it springs back up again but fluctuates for a few seconds before eventually returning to 13.8v position. This happens even when I turn the voltage knob on the PSU up full.

The more power I try to draw, the further down the voltage needle drops and, of course, I do not wish to damage my radios so I cannot keep the mic keyed. I tested it also with a 5w 2m handheld and the voltage didn't drop at all. But anything more (25w from the FT-2500M or 100w on HF or 6m on the FT-897) and the voltage drops like a lead balloon.

I have been told the problem might have been caused by modulation of my AC through RF from my longwire.

What I would like to know from the experienced hams and electronics experts out there is - should I find a deep hole to drop the PSU into, or can it be repaired? (Also, of course, can it be repaired cheaply and/or easily?!!)

If you have any advice that can help me, you can either comment on this blog post below or send me an email to hamradioireland (AT) gmail (DOT) com. Thanks.