What might be causing a 1991 Camry to shudder badly between 40-45 mph.

The car, a V-6 smooths out some at 50. It idles ok, and drives fairly good( but not great ) at speeds below 40. I recently changed out a bad fuel injector( failed the ohms test badly ) after having almost identical symptoms, and it ran fine after that for several weeks, now this. The injectors appear to be working ok ( strong ticking at idle when checked with stethescope, ohms out around 15-16 ) near as I can tell. When the vibration occurs, if I tap the brakes or put it into idle, it pretty much goes away. Thanks for any help.