How big of a heatsink are you using ?
I had two drives running on a half inch block of aluminium about 8 inches square that had two CPU fans bolted to the opposite side, and this worked well, the whole lot got warm but no burn outs.
Also the IRFZ44 is a MOSFET and is a different beast to the TIP122's which are transistors.
Better to stick to the design and possibly look at bigger heatsinks or better mounting, I used the little rubber like insulators on mine and this seemed to work well. The TIP122's should be able to handle 1.2 amps without problem.