if pressed for a quick stab, id say MAF or ECT sensor - either can cause weird fuelling issues that seem to occur with a temperature connection. Mine used to run fine cold, then would lose power and backfire as soon as it hit 60oC on the MFA oil temp, plunging bucketloads of fuel down the exhaust. 19mpg at 40mph it was doing!
The MAF showed "short circuit to gnd" fault, I replaced it, hey presto. 33.5mpg at 65mph and only backfires now on the over-run, as I have a decat pipe....
BUT you need the codes scanned first.
