Are both same year? They improved the fan at some point, I think like 93 or so. My 92 has the old slower fan.
I think the fan motors are single-speed and I'm not aware of any malfunction that would cause it to slow down, except possibly a very bad electrical connection that starves it of current. You might try checking the wiring.
Nonetheless, the faster fan would be an improvement, so you could replace yours with a newer one. I've been meaning to do this just on general principles, but the old model is sufficient if everything else in the cooling system is working right. If you're having overheating problems, I doubt a faster aux fan alone will fix the problem. Check some of the recent overheating posts for a list of other things to do to fix overheating.
