Hello all, first off i have a 1998 A4 1.8T Quattro

So A few weeks ago my car started to seem to lose some power when i opened her up a little (no pun intended...) right around 3k rpms thru 32k. I thought nothing of it rather than spark plugs plus my CEL came on. So i got home, went and got some new Iridum plugs, swapped them out although they did not seem dead nor "bad" it had been about 10k since i last changed them and so i did. I thought the problem was fixed cause as soon as i turned on the car the CEL went away. I did not drive her very hard the rest of the day. The next day on my way to work i opened her up and the same problem AGAIN. The CEL did not come on for a few more days but now every time i want to gas it a little harder, the car kinda shakes, theres some struggling almost gurgling if you will, it's still between 3k-35k now. BUT, after it passes through about 400-600rpms it clears up and at 42k it opens up nicely. I had a CEL a while ago (Code:P0420 Catalyst System Efficiency Below Threshold (Bank 1)) but it would come and go and no performance trouble was noted. So any ideas on what i can try or do? Could it in theory be my CAT? Oh, i am about 2k behind on my oil change and i did add about 2 quarts around the time it was due for one because i thought it had burned through almost all 5 quarts. AND to add on top of all this i took it to my friend to get an oil change (he lost a bet) over at EZ-Lube and one of the lesser knowledgeable people said that since the last oil change (which THEY also did) some over tightened my drain plug and in the process of trying to remove it this time they stripped it. So i have still yet to complete my oil change and cannot until this weekend.
Thanks for any advice i really appreciate it. Its stupid to say one never has time for their car but full time work and school make it seem like theres never enough time. Thanks all

Eddie, San Diego, CA