1998 Chrysler Sebring misfire

1998 Chrysler Sebring
6 Cyl.
119000 mi.

Just bought this car for my Daughter and have had the following done with no results. (Paid $1500.00, but put $695.00 on sales slip to help with sales tax, got boned)

Replaced plugs, rotor, dist. Cap.
Checked out misfire Cyl 5, codes PO300 & PO305 found.
Leak down test (Good) for Cyl.5
Compression Test 172
Replace lower and upper intake Gaskets
Swap Injector & Spark Plug 5 & 6 No Change.

I also had to replace the Cat. Converter because it was broken up inside, the guy at the Garage said that could have been from the injector at 5 shooting to much gas and overheating the Cat.
Garage said the computer seems as if it was replace at some time because there is a sticker on it that says it has to be programed and New cars do not have these stickers.
He suspects a wire could be the problem but says it could take days to find it. I'm already into this thing for $1200.00 (parts & labor) on the of the $1500.00 I paid. Could someone point me in the right dir. If I were to trouble shoot the wiring myself. I do HVAC so I do know something of wiring.

Thanks