Electrical problems

I have a 1995 Beretta with a 2.2L engine w/ auto tranny with 115695 miles.

I have a problem with the cars electrical system. Just replaced an alternator when I noticed that the air conditioner was "surging", that is, the fan would speed up, then slow down intermittently.

Driving the car back home at night, it was doing the same thing, and the headlights would flicker.

Fearing that the lights would go completely out, I shut off the AC, but the lights kept flickering.

Stopped at a parking lot to examine under the hood, while the car is at an idle, everything works fine!

Getting back on the road, under speed, these symptoms reoccur.

What the heck is going on here? I've always had problems with this cars electrical system, I keep changing out bad alternators as something seems to doom them all to a early death.

My turn signals will quit on me too at various times driving the car. And it seems my rear window defroster takes a heavy drain on the system causing lights to dim, fans to slow down, flashers stall, etc, etc.

Cold weather REALLY makes these problems even worse!

Any help to resolving this very irritating problem(s) would be most appreciated!