Can't accelerate past 45-50mph

Hello... Name's David.  Have an 04 Grand Prix (non super-charged)... and can't figure out why she doesn't want to accelerate past 45-50 mph!  It started a couple months ago while on the hwy uphill, when suddenly the correlation between the gas pedal and the speed in which i was going were no longer in sync! I could press the pedal all the way to the floor (most of the time i still can) and the greatest the car can achieve is around 45-50 mph at about 1500rpm.  At the time of when it first started happening a resonating hum was beginning to bellow throughout the car and the check engine light came on.  Took the car to Autozone to get the codes for the check engine light... bad Catalytic converter. Replaced and hoped i'd get a quieter/faster car.  The only thing that was remedied was the check engine light was turned off.  Still the loud hum and still limited to 45-50mph.  Went to a shop and explained what was happening... ended up replacing both front wheel bearing assemblies in hopes that the sensors in the assemblies might be the culprit.  The loud hum was finally gone and i could finally 'hear' the car again... but still... can't do any highway driving!! A week or so later when i started off to take the car to a transmission shop, she started working and got me there at 70mph?! So for the last week or so my car sometimes works like she's supposed to, but most of the time i'm stuck making my long commutes on feeders/ frontage roads/side streets! When accelerating up to the 45-50mph the car responds normally transmission-wise (sounds, feels correct) while the tachometer reads correctly before dropping to 1500rpm at 45-50mph.  When she decides to work, the tachometer/gas gauges sometime read the way they're supposed to... while other times the gauges all flat-line minus the speedometer... but the car feels, sounds, responds the way it's supposed to when she graces me with highway speeds! This makes me doubt it's something mechanical in the transmission...Is this something electrical like a faulty relay or wire? or something else entirely? Spent sooo much money fixing the wrong things and before i spend more just for a shop to 'look,' I'd figure i'd ask for help from somebody who could point me in the right direction!!  Thanks for taking the time to get back to me