Zorro listen. You've got a corroded chassis ground in your engine compartment that's causing your power problems.
There are two that are easily accessable in the engine compartment -- it will be the one on the driver's side as this feeds the aux fans, although both should be cleaned and retightened at the same time. The connection can be intermittant but usually will fail if you put a high load on the connection by operating the ac and headlights at the same time. You may notice your left headlight (drivers side) as being noticeably dimmer. You may also notice that operating the left turn signal puts enough additional load on the connection to make it fail. It simply will not pass enough current to operate the aux fans reliably.
Find the chassis ground point on the drivers side in the engine compartment. This will be several brown wires with ring terminals beneath a screw into the chassis close to the drivers side headlight (if I recall correctly). Remove the screw and clean the ring terminals and chassis connection with fine sandpaper and then reinstall. This should cure your problem.
One final note: there are two sensors that operate the aux fans-- one is a temp sensor on the engine block and is not the same one that operates your temp gauge on your dash; the other is a high pressure switch on the ac system. If the temp sensor has not been replaced then go ahead and replace it also. It can be intermittant and this is cheap insurance.
Oh, and I highly recommend a new mechanic.