Sounds like the crank sensor to me.
If you want info on testing the crank sensor I could get you some info, you'd have to get a multimeter, and make a little LED test light, you'd want to test it when it is failing.
Check vitals like rjolly suggested, then if you determine it needs replacing, and you want to replace the part yourself, then here is a thread that will help.
Change crankshaft position sensor
A off topic tip for the future, if your Dad has never had the lower intake manifold gasket changed, I recommend planning to have the LIM Gasket replaced with the aluminum framed Fel-PRO LIL Gaskets.
The nylon LIM Gaskets are known to fail, and can lead to a hydra-locked motor, and possibly a destroyed motor.
UIM and LIM gaskets L36