Gas path offers a wide range of potential causal factors, all of which are wholly relevant, but also hits the nail firmly on the head with the two prime causes--filter change--or lack of to be more precise--and ice due to water. Water drains are there for a purpose and I admit that, whilst I did not enjoy the task I always did it thoroughly---and with all due respect to aircrew here--you would be amazed just how much water can get drained at times.
As I recall, Quantas always insisted a sample be shown to the Flt Eng as part of his walk round--which shows how much emphasis they placed on this particular and avoidable problem.