I am experiencing an issue with my 04 GT. When the car is driven off a cold start the issue is nonexistent but as it warms up it wants to shift early when at low throttle input. I can be going 30-40mph and it seems to shift into overdrive, my vacuum drops to around 3.8-4.3 and there seems to be a large dead zone in the pedal response. If I tap the pedal or apply more than half throttle it will downshift/rev up and take off as normal. It almost feels like I've lost all power until it downshifts and seems to want to shift into OD earlier as the car gets warmer. I have a hardwired radiator fan, temps stay around 195-200. The car has thrown an intermittent P0420 Catalyst Below Efficiency code but it doesn't always come back after being cleared. I never smell any fumes or Sulphur smell but upon changing my rotors in the rear noticed both mufflers have 5-7 holes on top of them. The car was purchased for $800 with 160k from a friend so I don't want to throw much money into it. So far all I have done is a general tuneup : plugs, wires, air filter, MAF, MAP, PCV valve and oil change. I will be doing transmission filter and fluid this coming week. I can't hear any intake leaks or find any spraying cleaner around.
Another weird symptom is the car seems to rev as I let off the throttle or brake while cruise control is on. Ex: Driving/Cruise set and at 1200 rpm, let off throttle, rpms shoot to 1400-1500 and then drop, is this just a symptom of the TC being locked up?
Any help is greatly appreciated before I throw anymore money at this thing or take it in.