Is It time to get a new car? help

I have a 2004 chevy cavalier, we started having problems after we got a oil
change at pennzoil ( we also got a air filter and oil filter, along with checking
all fluids) the guy at pennsoil said it appears you have a little water mixing
with your oil. what? anyway my husband was going to work and he said
when he stopped at a red light the car stalled but started again, then the
check engine light was blinking. got a friend of ours to replace a part by the
muffler it was making a racing car loise ( so loud) after he changed the part
by the muffler the check engine light went out. goody. well a few days later
he was going to work and the car stalled again, our friend said when he is
has to stop and sit at a light brake but at the same time give it alittle gas this
did help but he said it shook a little like it wanted to stall. He made it home
and our gas gauge was killing our gas we went through a 1/4 of a tank for
him to go to work and back very impossible. whats going on? so our friend
came back and now the check engine light came back on but it wasnt
blinking it was solid. so our friend looked at our fuel pump and said it was
good since we just had it replaced 3 years ago so he decided to change
our fuel filter still didnt help so now we are lost people are saying it could be
a catalytic converter head gasket. anyone have any idea what is going on
we are at 133,000 miles. money is tight also.