problem dont have a clue wats wrong

hello guys,

for starts i have a 1999 audi a4 quattro 1.8t 150,000 miles

now heres the problem and i think i caused it i just dont know what i did so here it goes....I had a exhaust leake at the flex pipe 2 months ago and replaced it with a straight pipe i know now that was a bad idea cause after that the large muffler looking thing in the back started to rattle so i decided to tie it off so it didnt do it anymore then the big problem happened the catilic converter snapped off of the flange that was attached to it so i thought i could just weld the cat back to the flange i did this with the battery unpluged and it still in the car now the weld is holding but when i went to start my car it starts then the rpm's drop realy low and the car shuts off. if i rev it up a bit it during start the rpm's will drop then go back up etc.. until it comes to a normal level i have then drove it and it does not feel like it has any loss of power but when i stop again at a red light or stop sign the car's rpm's drop and do the same thing where it levels itself out or it stalls out. my check engine light isn't on when i have the car running it is only on after the car stalls out also when the car's rpm's drop before it stalls my battery light flashes as well but neither light are on once i get it running any help would be greatly appreciated