HELP!! Au xr8 series 1 Running rough,

[keshin]

hey guys in need of some help here,
i have an au xr8 manual, a week ago i was on a cruise and she was running fine as it has since i have owned it (over a year)
later that day i turned it off for about 10 mins then when i started it and got back onto the road it started running rough, backfiring through air box, (feels like missing)
I have ordered an elm327 to see if any fault codes are showing but it probably wont arrive until tomorrow
Car has done about 210000 k's
since having it at home it seems to rougher on idle than it did when it started playing up.

Notes:
*i have replaced the fuel filter since this happened
*air filter is not very old and clean
*i pulled maf sensor out and it looked clean (no corrosion or brown color)
*the car had new cats fitted about 6 months ago
*i have never run the car dry (out of fuel) but have had it low
*the camshaft syncronizer has not developed any squeaks or noises
*I have unplugged the ISC and the car still ran until i gave it a little rev then it died out
*the plugs, leads, coil pack do not appear to have been replaced ever
*the exhaust seems to sound alot more crackly and has some higher pitched popping noises
*i have run the car in the dark and cannot see and sparks etc. from the leads.

Im after suggestion from anyone who has any idea what is causing this, i would prefer not to go replacing spark plugs, leads, coils, pumps etc unless i am pretty certain that is the problem. I am out of work so funds obviously are a little tight at present.
Thank you guys in advance