Have you measured/checked the supply voltage?...when first turned on as well as during operation as it becomes "anemic"?? Not sure what your power source is, but this might provide an 'upstream' problem clue.
Since I believe this version of the pump has an electronic timer board...instead of the old bimetal strip...if you can probe the coil leads at the circuit board at start-up and during operation, it might reveal a circuit/component issue. (The EE geeks among the forumites might have a better clue re this possibility.)
Also, have you manually checked for free motion of the plunger after voltage has been applied?...as it becomes anemic?? If the coil is getting overheated, perhaps it might be affecting the free motion of the plunger.
OTOH, not sure how old this accessory is, but there are days I can relate to the symptoms!! My mojo ain't what it used to be!! And Geritol probably won't help your pumper, either!