Greetings
I am hoping that one of you mechanical geniuses out there might be able to help me figure out why my Specialized Ruby has a dangerous quirk. I find myself stopping frequently because the rear wheel gets that squishy feeling like the tire is flat but the tire is fine. A casual coast down a hill and the rear wheel will feel like it's wobbling irratically - sometimes severely enough that the rear wheel jerks out from under the bike violently and unexpectedly (always jerking to the left). This has led to several crashes and somesevere injuries. Ive taken the bike in to the mechanic repeatedly- everything checks out perfectly. I've had everything from the drive train to the rear wheel inspected and replaced. Recently a fitter moved the saddle forward almost 2 inches- effectively moving my center of gravity forward on the bike. The problem of the rear instability got much worse. Now either i've pissed of a possessing bike demon or i have a bike frame that becomes dangerously unstable when its front loaded.

I'm toying with the idea of getting a custom frame made. What details should the builer and I address to resolve this issue?
Many thanks for your ideas