car help

I purchased a 1999 Dodge Durango after my Jeep was destroyed in an accident, I had to put $500.00 in the tranny first thing I am planning on putting on a complete new exhaust system I think the converter is clogged
but the biggest problem I am having now is that it will not stay running after it starts I have to wedge something on the pedal to keep it running and as of late it is starting harder I know that the 318 is prone to sludge and alot of other problems but I cant seem to find answers on this one PLEASE HELP I am at the end of my rope with this

Sounds like it could be a problem with the fuel filter. Since you just bought it you should change all the fluids and filters so you know for sure that they're nice and fresh.