06 Stratus Grinding

I've done quite a bit of work on cars in the past, but this one has me stumped. This is for an 06 Stratus.

I am getting a periodic grinding sound and loss of power for acceleration about every 25 seconds accompanied by dimming of the headlights and dash. This only occurs when I have the heat turned on to either the defroster or defroster / floor setting.
I was originally thinking it might be a transmission problem - it sounds similar to me - but it occurs at any time; stop lights, accelerating, decelerating, even when the car is in park.
I checked the RPM meter for behavior while this grinding occurs - it reliably shows a drop in RPMs followed by a spike, after which everything returns to normal.

Any suggestions for what I should look for?

Update - I've taken the car in to the shop. They have diagnosed this as a problem with the air conditioning compressor.

Their explanation makes sense to me.
If the clutch is kicking on periodically to engage the compressor that would account for the periodic nature of the problem, and if the compressor isn't moving freely that would explain the drag.

Instead of paying to have the shop do all the work - they're asking a couple thousand dollars - I'm thinking about doing some of the work myself.

I would have them discharge, flush and recharge the system but do the actual compressor and clutch installation myself. That way all the refrigerant stuff a professional is legally obligated to take care of is done by them, but everything else I could do - it can't be any more difficult to replace a water pump or brake calipers, right? :P

Also, does anyone know if the clutch and compressor are sold as a single unit?

I had the same problem with mine and it was fixed with a minor adjustment. Not a Stratus but had the exact same symptoms. A simple adjustment to re torque bolt down to specs fixed the problem and hasn't returned.

you can get the clutch assembly by itself, if you buy the compressor it comes with a clutch