Could be fuel starvation... Had a similar problem with an old Vauxhall years ago, the cause of which was discovered by pure chance - on suspecting fuel starvation, I removed the pump (piston-rod type, driven by an eccentric on the camshaft) and manually operated it - full stroke pumped what looked like a good solid squirt of fuel. On the car, however, the rod was jamming 1/3 of the way out... And only spotted because it was mounted right next to the oil filler cap and could be seen working (or not, as it happened) with the engine running and filler cap removed!
Long story... but one with a point. If you have, or can get hold of, an inline fuel pressure gauge, test the rail pressure at idle and under load.
Also look for vacuum leaks. A simple OBDII code reader should give you any generic codes stored in the ECU, among which, IIRC, are 'lean mixture' flags that may not necessarily throw a CEL.
