Mine did the exact same thing - bad alternator. I rebuilt it and it's been fine since. The wipers would go really slow too - you may want to look at those as well.

'91 LWB, 31x10.5s, KYBs, A/T "I look like Robert De Niro, I drive a Mitsubishi Pajero" - Billy Bragg (not the original lyrics, but that's how he sang it when I saw him in Dublin in '04)