1995 Chrysler Concorde erratic idling

Car starts fine then if I rev it up slowly it starts erratically climbing, when I put it in gear it backfires repeatedly through intake and is very hesitant also cruise control light flips on and off as does the check engine light when in motion. Died once at parking lot speed.
it ran fine yesterday, I changed oil last night using mobil 1 10w30, a fram high mileage filter and lucas oil stabilizer I put a little over 5.5 quarts including oil stabilizer. Started it after oil change and drove to store it was fine then, but today is when problems started.

car has 3.5l engine and abs

23 0ct 08
i was just informed about turning the key on and off 3 times and keep it on the last time and the check engine light flashed as follows 1.2.4.3.5.5

25 Oct 08
replaced ignition coil pack still same problem

31 Oct 08
had a mechanic spend four hours going through it and replaced fuel filter and spark plugs no positive results car acts the same. The mechanic was completely stumped as to what is wrong getting no codes and everything seemed fine by scanner and checking all vacuum lines and all connections.
(scanner says car is okay to drive) mechanic thinks either computer is bad or timing is off slightly (doubts timing because car runs to smooth at idle, startup and killing seems normal).

01 Nov 08
today car overheated, I think the thermostat froze shut is it possible that my whole problem can be vapor lock or a cooling problem? Will replace thermostat and coolant temp sensor tomorrow.