misfires/holds back at freeway cruise speed

When I am cruising 65 or 70 I feel the car holding back or erratically missing. Whe I accelerate it smooths out, but as soon as I try and keep a steady speed it starts the herky-jerky thing. Check engine comes on every now and then. 95 mustang gt, 5 spd.
Around town, stop lt to stop it's fine.
Any suggestions?