I have a 1993 silverado with 118,000 miles. It has a misfiring problem(breaking-up) when bringing up the RPMs. It occurs when you are easy on the accelerator or let off the accelerator and then accelerate easy. At higher speeds the problem goes away or flooring the pedal it does not missfire. Also, during backing-up you notice a roughness or miss from the engine. It has a new timing chain, fuel filter, plugs wires and distributor cap. I am at a loss and need help, so please help! What does it sound like?