1995 Geo Prizm Runs rough and stalls at stop lights

My car just one day decided not to start and had to tow it to the neighborhood auto mechanic.
There they got it running with a full tune up and overnight they could not start it the next morning.
So they said it was a fuel injector and changed that and now told me it was good to go.
Well the next day it did not want to start again and when I finally did get it to start it stalled at all the lights and stop signs.
Now I took it back to them they could not find anything wrong and set the idle up.
Today it did not want to start again and runs rough when it finally did start.
I can, t afford to put anymore $ into it.
I, m into $600.00 now.
Could a clogged catalyst converter be the problem if so how do I check for sure.
Like I said before I cant afford to put anymore $ into it unless it is the fix.
I, m unemployed and no income and looking for work.
I want to get my car back to the reliable car it was before all this starting to happen.
I do smell exhaust coming from under the hood if that helps to narrow the problem.