highway power sluggish on my 98 chevy C1500
 

hi im new here. . .well i bought me a 98 chevy C1500 pickup with 350 5.7 vortec i had some flowmasters installed don't know what they called but it had 2 pipes in back of muffler with 2 pipes going out. . .the problem is when im on the highway the truck feels sluggish in the sense that i step on gas pedal but it don't feel like it picks up good. . . the rpms go up and it goes to 70 quick but don't feel like its going 70 cause i see people passing me up that were way behind. . . .i changed the spark plugs and don't know if thats the problem i put autolites and i had to go back and exchange one cause it was defective so im thinking of buying more expensive spark plugs to see if that will give it more power. . . .or could it be the oxygen sensor might need replacing. . . .what can it be and what can i do to get more power out of the truck

Try cleaning your Mass Airflow Sensor. The speedos in these trucks seem to be somewhat optimistic, mine reads about 2mph more than true speed. Remember you are driving a heavy vehicle so don't expect lively part throttle response.

ok cleaned it put some new acdelco spark plugs got new pcv valve but haven't tried the truck yet but thanks for responding