What kind of air filter did you get?  Is there anything caught up in the filter or the piping? 

sounds like your in 3rd gear to me. i cant think of any way that an air filter could do that.go from a stop and count how many times the car shifts on its way to 80 or so. or hell,try to punch it when your running 80 and see if it downshifts.id almost bet its in third.

even if your torque converter isn't locking... you only lose like 500-800 r.p.m.s through that... you are definatly not shifting into 4th and staying in 3rd... The only way I can think that you could have done this by replacing the air filter is by somehow knocking the plug on top of the trans... but i doubt this...

I would check your O.D. button on the gear shifter... that is probably the problem... either it is pushed out... or the button stopped woring... or something along those lines... don't mess with the torque converter unless after you get it into 4th... and it still won't get down that extra 500-800 r.p.m.s