defective exhaust?

[Festively Jolly]

hey guys.  i got a new problem.  i  replaced the exhaust on my 1988 Audi
90 FWD with an oem. (blah).  but it is all new cat back except the
muffler.  well when i start the car, hot or cold, it will run like crap.
i don't know if that is  the exhaust or what but the noise... oh my if i
put the gas down within 5 min or so of starting the car, it sounds
horrible! kinda a metallic fizz. its bad though.  then i "heats up" or
SOMETHING and it sounds a lil better. BUT if i run the car hard, it will
give a few backfires i think.  im not sure, but when i let off the gas
it will pop a few times. nothing LOUD LOUD, but enough to degrade the
sound significantly.  i love the engine sound, but the exhaust is bad.
can i take something off?  add something? replace?  thanks for the
help!! later