Flashing CEL .... Rough idle ....

Ok so here goes ...
i got a 2000 dodge caravan Base 3.0 ... a year ago the check engine light came on, but no problems driving. Brought it to the Mechanic and he said that it was because i had a small leak in muffler ... so the other day my wife takes daughter down to beach, she gets home and tells me to jump in ... Van is vibrating really bad, no power when excelling and exhaust sounds really loud ... Go around corner to buddy's house (Because he has tools) we crawl up under and see a little slice down the muffler, put aluminum sheet fitted around the muffler with pipe clamps, and seals it so you can barely hear it now. Start it up and still vibrating and no power. Go around corner to bring my van home and the CEL starts flashing. Get it in driveway and shut it down. Borrow a OBD and get codes (Some most likely from before) but here are codes in order they given PO303, PO300, PO420, PO302, PO203 .... Any clue would be a big help ... i dont really have money to let a mechanic try to figure this out ...
Thanks Matt

If the CEL light is flashing that indicates a misfire.

The flashing CEL indicates the misfires are so bad you are damaging the cat converter. Get it fixed before you wind up forking out $500+ for a new one.

http://www.obd-codes.com/p0300
http://www.obd-codes.com/p0203
http://www.obd-codes.com/p0303
http://www.obd-codes.com/p0420

203, 303, 302 are causing the 300 and 420. For starters check for loose spark plug/fuel injector connections at cylinders #2&3. If the plugs and wires have been in it for several years consider replacing them. #2 is at front left and #3 is middle back side.