I have a 96 tahoe that has been running rough off and on. Traveling @ 45-55 light on the accelerator with little to no load/strain the engine "bucks" and the RPM's jump a couple hundred then returns back down to where it was. Occasionally the service engine light comes on. Sometimes it doesn't. Been to the dealer 3 times for a diagnostic. Nothing identified. Almost acts like it is trying to downshift. Any advice?