I may be being thick here--but would a new Piston Rod
http://www.airgunspares.com/store/pr...art-No.-STD13/ cure this or is the problem still with the sear? (given the sear worked in another block???) I think a few of the the tuners and engineers on here can make up piston rods so should be able to sort something out (either making a rod or building up with a decent welding set up and reshaping the notch on the rod)
I would agree with not trying to squeeze any more power out of the gun by fitting a stronger spring. With an old and worn gun, sometimes with a bit less power/ spring they can function well in the less streseed state but that little bit more can shove them over the edge of reliability.
It is interesting to know about the Mk3 sear.
Reading this through, it does seem that is needed reducing to the basics of piston/rod and sear to find the problem, rather than looking at other things.
At least you have a working gun now