I have a 2003 Ford zetec 2.0 l four door car. For a while now I've been having an issue with the vehicle losing power and the check engine light coming on whenever I push the AC button or sometimes flip the headlights on. Sometimes the issue can be mild and caused a minimal loss in power sometimes it can be extreme causing the car to rapidly downshift into very high RPMs. This can happen while underpower and accelerating or hardly any power and coasting down a hill. The other thing that happens is that I immediately lose power to the radio when this event occurs . Everything on the internet said to replace the alternator so I have done that. This helped the problem but it did not resolve it . I then thought well maybe it's my spark plug wires or my ignition coil. I have replaced all of this now with very little change. I have had my battery load tested as well as my alternator while on the vehicle with it running. This was done at O'Reilly's Auto Parts store. The thing that really gets me is that it seems to have such weak electrical signal that sometimes when the windows are already rolled all the way up or already rolled all the way down if I keep pushing in the direction that they already are it seems to intensify an issue that you can see a warning bulb through the dash. I have gone through every single ground and made sure they were bare metal under the hood and applied Dielectric grease when reattaching the bolts. I then went into every electrical connection that goes to things like the mass air flow sensor and sensors on the engine and opened them all up and applied a very small amount of Dielectric grease on the tip of a toothpick to every pin in those connections. Nothing seems to fully cure the problem nor can I completely diagnose the problem. It would be wonderful if someone would chime in that's had a similar issue. The car has 163000 miles on it and it's just a daily commuter vehicle for work.