In that case the block learning is taking the settings away from optimum for some reason. IMHO, could be a poorly Lambda sensor or an exhaust leak / EGR / SAI valve fault causing it to read inaccurately.
Worth checking the rest of the engine management inputs to make sure they are OK too (MAF and coolant temperature, throttle position, etc).
Is it throwing any fault codes or just running rough?
Kevin
It doesnt throw any faults, it just gradually starts to run rough.
EGR is blocked off and has been since owning the engine with no problems for well over the 1st year of running, had both oxygen sensors replaced, exhaust leak has not been something i have checked, how far back towards the back box would a leak cause these problems? SAI, not clued up on this im afraid, what am i looking for?
Had new CTS, tried 3 different MAF's and have never replaced the TPS. Can the TPS be causing problems without throwing codes?