Hi,
So you can see the problem with Fritzy.
You have 10uF caps on the input and output of the regulator, as close to its pins as possible?
I would guess that your problem is position of wiring and age of your valves, if it has worked okay for so long.
Keeping the 24Vac well away from other wiring may help.
Definitely fit some snubber circuitry.
Can you isolate the fault to one particular valve?
Thanks.. Tom... 
