For the past year or so going down the road around 60mph on flat roads the engine would surge about 200 rmps. Yesterday when I drove it I noticed that at 60mph the engine was running around 2100 rpms. The truck used to be down around 1500 rmps or so. Later that day the check engine light came on. The code it gave was p1704 which was the speed input sensor. I changed that and still no difference, its still running almost 600 rmps more than normal. I had the tranny rebuilt around 10000 miles ago becuase it would not shift down. That was fixed and the mechanic said the clutch plates and everything else looked good. What could cause this? A bad torque converter?