Mosfets and Microprocessor

I tried to simulate stepper motor drive in Proteus ISIS;
If I directly drive the motor by the controller it works.
But If I add a mosfet (IRf44n) in between as a driver; then it errors: time step too small.
I am sure the mosfet is wired correctly.
I hope someone knows whats the matter.

Bad Mosfet model, most likely. This is the reason I like LTSpice. Mike Englhart, the guy that develops it for Linear Tech, has fixed most of the bugs in the underlying Spice computation engine to where I do not get the "time step too small" error. When I used earlier incarnations of SPice, I used to get that error all the time.