You should probably completely replace the fan, if it ever fails completely the unit will self destruct, and the fact that it's been run when it's slipping hints that it's been run over temperature, a safe bet is replacing it won't hurt and will likely help, even if you decide to implement a dimmer on the main bulb. Mind you this being on a projector with other frequencies being produced flicker may occur. The flicker of a halogen not being noticeable is only from direct human persistence of vision. You might see subtle banding depending on how the LCD of the projector is updated.
I didn't think about the lower temperature of dimming a halogen would shorten it's life, but after Hero's post it seems obvious, because the 'regeneration' of the filament doesn't occur at a lower temperature when the element isn't 'gassing' to redeposit itself. I don't know if the reaction is actually evaporation or sublimation but I always found halogens an elegant upgrade from traditional incandescents.