92 cadillac deville 4.9l engine

i know its not a honda but im trieing to fix this car so i can have my honda as the weekend driver.. i bought the car for $450 and it was a little slugish. i got a new cat and the muffler taken out with a straight pipe.. it didnt help much. so driving home this was a week after i bought the vehicle.. when i came to a stop light the car died.. and sometimes it wont start.. i replaced the fuel filter. so i got a pressure tester.. and i read 4psi key on no start. and with car running it goes to 35 and drops down to 20psi after 30 seconds of the car running.. i switch off the car and it goes down to 0 psi after a 5 seconds.. when i unplug the vacuum line on the fuel pressure regulator the psi on the guage goes up about 6 psi... any suggestions or tests i should do next.. so my symptoms are
car wont start sometimes
takes 5 times turning the key to get it started.
car runs rough and idles at 1300 rpm hot or cold
dies when coming to a stop
no power after a minute of driving wont go past 10mph.