not sure how to explain it cuz its my bro's truck. he's complaining that after driving for about 30 minutes or so, he would notice the truck to slow down after reaching 40mph and beyond. he said "it would feel like im driving with parking brake on". I tried to drive it but didn't have 30 minutes to spare cuz i was late...so i couldn't feel what he was talking about. also, no codes popped up. do u think a transflush would help this problem. he has a 2004 frontier 4 cylinder w/ about 60K miles. only thing done to it was changed oil and air filter about a month ago, replaced serpentine belt couple of months ago.