Having some issues with my 2002 Impala with the 3.8 6cyl engine...about 66 K miles..
1) Slower than normal acceleration and lack of pep.
2) Under heavy (pedal to metal) acceleration cannot get past 65-70 mph. However, if I accelerate slow no issue.
3) Check engine light flashes at about 65-70 MPH but goes off when I decelerate.
4) Also under heavy (getting on expressway) acceleration shift into high gear is delayed and hard (at about 40-50 mph). No issue under normal acceleration. Tranny shop did test drive and said probably not tranny.:banghead:
5) First start in morning a little slower than normal.

Any thoughts on what could be issue here? or more than one issue?

if your cel flashes then it should have a code stored under history codes, have it scanned.

Aslo have the fuel pressure tested and get a back pressure test done on the cat.

I had the same problem on an impala. I drilled a hole in front of my cat and drove it. Everything was back to normal except for the loud exaust. I put an after market one on from champion for about $90.

Sounds like a classic clogged CAT...