Hi, my 2003 Chevy Avalanche 4x4 Z71 with 140k miles is up and down shifting around the 3rd and 4th gear mark. It will up shift and down shift quickly, almost as if the truck has a governor on it, bouncing between the gears quickly. The speedometer moves erratically above 40-45 mph or so, jumping 15-20 mps increments all the time without affecting the actual speed and performance, the rpm's remain stable at that time. The rpm gauge will also jump at times, with an up or down shift in the gears, usually between 3rd and 4th gears. I've had diagnostics run on the truck and many codes came up. I suspect the check engine light had been burnt out before I purchased the truck 5 months ago, because there was a code that the check engine light was on, yet it wasn't. The up and down shifting between 3rd and 4th is becoming more and more common. I changed the spark plugs to AC Delco plugs, and wires, the speed sensor on the transmission, the fuel filter, the air filter, and the brake pads and rotors. Yet, the problem still remains. They needed to be done anyway because of the care that had not been taken during the previous owner's time. The codes from the diagnostics read: Service Brake System Light on; C0237 Rear Wheel Speed Sensor Signal Erratic; P0300 Random Misfire; P0135 Bank 1 Sensor 1 O2; P0155 Bank 2 Sensor 1 O2; P1875 4 Wheel Drive Low Switch Circuit; P0894 Excessive TCC Slip. The Service Brake System comes on when a suddle thud is heard from the rear of the truck, yet this could be something with an alarm going off that isn't heard because the stereo system is after market and likely bypassing the chimes/alerts that the truck sends out.
Also, en route diagnostics were performed while the truck was in motion, driving down the road. The speeds from the front left and right were steady, while the rear spiked up and down constantly as the speedometer fluttered.
Any idea what is going on? Torque converter? Throttle position sensor? Bad speed sensor at the rear?