Voltage drop

I have problems with my car seems that the voltage drop, in the mornings I have no problem to start it, crank it a little but starts at the first try, but when I driving slow like 20 mpg, the air bag light start to blink and the temperature gauge start to drop, the electric windows work very slow, looks like the car is going to died but don't, but when I press the gas looks like everything is normal, in these conditions if I turn it off, it won't start. Also the headlights are very dim and the lights in the dashboard too, when I press the gas they are very bright. The a/c compressor is not working. A mechanic said that the fuse and relays are ok. I already changed alternator (rebuild), new battery, driving belts, alternator belts.