As we already explained in an earlier thread, you can't have *parameters* in a simulation that are time-dependent, even less so depending on a node voltage or on a current. Parameters can only be constants for the whole simulation; the simulator can step parameters over some range though - in this case, it will just launch as many simulations as there are steps, every time with a new constant value for said parameters. Not at all what you want here.
Also as we explained earlier, you can use an arbitrary voltage source for this. A sin function combined with some thresholding function could get you a voltage-dependent pulse width generator.
Alternatively, you can look at one of the "modulate" models in the "Special Functions".