3rd,4th &5th gears--sometimes little power, sometimes lot of power when accel.

first thing that comes to mind is fuel filter and or fuel pump. a semi-clogged filter (or a weak pump) is generally noticed in heavy throttle applications and at higher speeds. if you can take you foot off the gas and the engine comes back to life, try by changing the filter (or filters) they are the cheepest and easiest.