Just because the CEL light is not on does not mean that there aren't any codes. Get it scanned. If you have access to a scanner that will give real time readouts from the sensors that would be even better.

I now suspect a fuel delivery problem. If you have a partial blockage or partial pressure problem this would allow it to run fine at lower throttle positions but give you problems at higher ones. And it may not trigger a CEL. The CEL is more related to problems with the exhaust. Not having sufficient fuel does not hurt the exhaust and so the CEL may not be triggered.

How old is the fuel filter? Note that IIRC there should have been a recall done on the fuel filter area. Also the fuel pump wiring is prone to failure.