98 F-150 5.4L 4x4, Under about half throttle starts to vibrate?

While traveling if I give it some throttle (about half) to accelerate it starts to vibrate but not seem to gain speed. If I punch it it takes right off and feels fine. Could this be a tranny problem? Torque converter maybe. Seemed to start out of the blue just now. Ran just fine last time I drove it. I have... show more While traveling if I give it some throttle (about half) to accelerate it starts to vibrate but not seem to gain speed. If I punch it it takes right off and feels fine. Could this be a tranny problem? Torque converter maybe. Seemed to start out of the blue just now. Ran just fine last time I drove it. I have installed a hypertech programmer, K&N fipk, and just had new fuel filter and spark plugs installed and transmission was just serviced by Ford. It really feels like something is slipping under half throttle and hooks up only under heavy throttle. I've been running the "firmer shift" setting on the hypertech for years. Could this have worn out my torque converter? Any insight would be appreciated!! Thanks.
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