Alternator
Classic symptoms that you have lost a phase in the Alternator usually due to a failure in the diode pack.
Check the voltage as others have suggested or take it to an Autosparky.

Belt
Belt squeal when cold will be unrelated (well 99% of the time) to the voltage/flicker.
Cold squeal is usually a belt on the way out or poor tension.
Don't use belt lube on a CRD, it may stop the squeal initially but it will come back and worse.