After driving for a few miles the engine will sputter/backfire and have no power with moderate to heavy acceleration (like its running out of gas). It will run fine as long as you accelerate very slowly up to about 35mph. After 35mph it will start to sputter even with no acceleration. Once the sputtering starts it will not stop unless you let off the throttle and it will also do it neutral when revving the engine. However once everything cools off the engine will run fine under moderate to heavy acceleration. It has been doing this every once in a while for the last couple of years, but now it does it all the time.