Engine? on 2004 Buick Rendezvous

I have a 2004 Rendezvous, yesterday I was driving and started losing engine power going uphill, This morning I changed the oil which needed to be changed, I took it for a test drive and it seemed the problem had gone away, 1 hour later as I was driving it did it again, I could only do about 35 MPH going uphill, will this be an engine, transmission problem? or changing the air filter will solve it, Please Help!!!

2 answers 1 comment
Another possibility is a failing fuel pump, unable to provide sufficient fuel pressure or volume. Is there a Check Engine Light/Service Engine soon light on? If so codes stored in the computer of your car may indicate where the problem is.
I have no lights whatsoever and it only does it now if I press the acelerator too hard if I go easy on it I have no problem and can do about 70 MPH
sounds like a clogged cat converter . a shop can do a snap throttle test with a vacuum gauge to confirm.

Roy