I am having an issue and it seems to be engine power related. I had a code for upstream O2 sensor, replaced both, no more code. When driving on flat ground it accelerates normally, get a higher torque load, such as a hill, starts shuddering &/or missing in the 3000-4500 RPM range and goes away before full throttle. Depending on the steepness of the hill, it may start a little sooner and last a little longer but always near 1/2-3/4 throttle and power range. Feels like a cylinder cutting out in that rpm range and whole vehicle shudders. Approx 135k, full service/ALL fluids done near 100k, (spark plugs incl). No codes. Also feels down on power in this mid range. Wondering about injectors or coil but runs good before and after that rpm range and on flat ground, thinking it should show up somewhere else as well. Also, this most likely is not a fuel issue as it has been doing this for a few months and many tanks, several places. Not ruling this out, though. Ideas?