626 shakes under load

I have a 1994 Mazda 626 with 2.0 engine with 204k. When on the highway at 55 up through 70 the front of the car shakes when under light load (when going up a slight grade). Strange, but it does not seem as bad at 75. Light load, light shake, heavier load, heavier shake - the more you press on the gas the worse the shake. On the level it is OK. I changed plugs and wires, checked cap and rotor - looked fine. Checked resistance on the primary and secondary windings of the coil - OK per shop manual specs. Should I change the coil even though it checked OK? Should I replace the cap and rotor even though they look fine?
I have checked for fault codes and there are none. This problem is not noticeable on back roads and in town driving, only highway driving.