V6 trouble at 1500 rpm

My 96 T-bird has only 37K miles. It has recently developed a jerking hesitation at 1500 rpm which mechanics haven't identified. Engine runs great at all other speeds. Fuel filter, plug, and plug wires all replaced. Transmission ruled out.

The shift point should be higher, but Ford built it this way, so it should run OK.

Any suggestions will be appreciated.