I have a 99 XLT 4x4 5.4 v8. My truck sputters really bad. It's popping or backfiring. A friend said its shooting sparks from the tail pipe. I give it full throttle and it has no power, it only gets up to 40 to 55 mph. Sometimes the idle goes crazy like when I first start it up it shoots to almost 2000 rpms and after that it just fluctuates between 500 to 1000 and will sometimes quit and idle normal. Not sure if it is related but thought it was worth mentioning. The truck also rumbles/shakes really bad for a second then goes to normal then just keeps going back and forth. I ruled out a clogged cat. I also don't have a muffler. Thanks in advance!