I own a 2004 ram 1500 QC and i just installed a mopar (K&N ???) high flow air filter and a flowmaster series 40 muffler on it. My problem is, when i hit around 47 mph the rpm's go down, the muffler quiets down, and it gains speed very, very slowly, this continues until about 60 mph at which time it takes off and everything goes back to normal. Is there a problem or is this "flat spot" suppose to be there?

 

I would change the TPS sensor, it sounds like it has a "flat spot" and is hunting for OD.