Short?

It might be that the condenser on the AC unit is going, causing a huge current drain and transient when the motor kicks in.  If your Niagra is doing its job, and the spike was generated over the wire, it would have been blocked at the power conditioner, which just leave airborne EMI entering the Mac.

There probably is a "run capacitor" on your a/c compressor. As pointed out in other replies, it is a sign that you might need a service call. If the fault only occurs when the a/c compressor starts and runs, then the trouble shooting should be easy.  Good luck. BTW, this should be a simple service call, perhaps, and I STRESS perhaps, you might be looking at a compressor, but I would get a second opinion for anything over a couple hundred$$$

Power conditioners don't restrict all noise coming from the outlets in the house. My Shunyata Denali allows noise through to my speakers when I click on my vpi record cleaner. Clearly audible. So your Niagara won't stop everything on the line. 

j-wall

Power conditioners don’t restrict all noise coming from the outlets in the house.

A good one should get very, very close.

My Shunyata Denali allows noise through to my speakers when I click on my vpi record cleaner.

What makes you certain that the noise is coming through the AC line?

How many amps is the sevice panel feeding your house?

What Higher energy needs cause the motor to pull more energy from your home electrical system. The result is a deeper dimming of the lights. When this happens, the AC will have to drain more voltage from other appliances in your home – thus causing your lights to temporarily dim. Eventually, the capacitor will fail and will need replacement.