Is there a possibility something is pinching your speedometer cable? Maybe your accelerator linkage (although that would be a stretch)? It sort of sounds to me like when you accelerate you're doing something that causes your cable to stop turning or to turn much slower. Possibly your rear motor mounts are bad and your motor (and thus your transmission) are lifting up on acceleration? Your motor turns clockwise, so you would be lifting up on the passenger side. If that is happening, your speedometer cable (where it connects to the transmission) could be getting enough strain that it stops turning as well.
