2000 Ford Contour 147k 4cyl

I purchased car approx 8 months ago and had to replace the alternator about 4 months ago. The car has run fine up until 5 days ago. I was driving about 50 mph with the AC on when car began to lose power (decelerate) and then regain, lose power, etc.
When I turned the AC and Fan off it quit doing this. I turned the the control to Panel and turned the fan on to high and it did the same thing. Later, I turned the AC back on and it ran fine. About a day later it did the same thing and has gotten worse. When the headlights are on and I open the windows they will dim for a second. If I turn the fan on, sometimes the ABS and brake light on the dash light up for a second and the speedometer drops down, then comes back and the lights go out. The car seems to run fine when there is no load from the fan or AC. Any thoughts on what type of electrical problem I might have or where it may be? Thank you for your assistance.