wrightd, I have to agree with Rover 'n Rugers about your problem. The GP should reliably shoot anything out there with the stock mainspring, especially if it's been "tweaked".
I am having some trouble with my 4" GP when shooting ammo with CCI primers. I slicked up the action in my GP but left in the factory springs. CCI primers are well known for being among the hardest to set off (if not THE hardest) of the readily available commercial primers. This led me to do a little looking and I discovered that my firing pin has a sort of squared-off tip and seems REAL short when compared to my SP101 and S&W M649, which also uses a frame-mounted, spring-loaded firing pin. Due to lack of cash to send it overnight to Ruger, I haven't called them or sent it in yet, but I will before too long.
It sounds like your problem might be the firing pin to me, too. I'd have a good gunsmith check it over and especially check the firing pin. You shouldn't be having FTF problems with any primers with the stock mainspring, let alone a heavier one.
Bub