06 chevy silverado v6 acceleration issue

I have a 2006 Silverado v6 4.3 , when I'm driving once ive
gotten to about 35-45mph I'll mash the gas to speed up and
it's ALMOST like nothing happens, Ill either mash the gas
harder but slowly and it'll start to pick up . now keep in
mind that if I'm going about 70, my rpms literally stay at like
1900rpms and when I mash the gas the rpms go up to
probably 2100 rpms, and if I mash the pedal harder it goes
straight to 3000rpms. The rpms and everything to fine up
until I get to about 45, sometimes it dont start till after 60. I
just changed my evap purge valve(engine light was on and
threw code p0496 which is what I replaced) I thought it
stopped it from doing whatever it was doing, BUT it didnt.
And also sometimes I have to let off the gas to make the
rpms to go higher once im going about 70.