I had a similar experience when I bought a used SB-600 that had light corrosion on the battery contacts. I'd clean it off and the flash would work, but it would come back if it sat for any length of time. I finally gave the contacts a good washing with diet coke and then alcohol and now no more corrosion problems. Make sure you give yours a good cleaning.
I use 2300mAh NiMH Energizers in my SB-600 and they perform well for me. I shot Santa photos at the local mall this Christmas, and flashed between 200-400 shots each day at ISO 800 and F5.6 with the head up at ~45 degrees and the defuser dome on. Between kids I leave it in standby mode and the recycle time was snappy all day long.
"I should have been a cowboy. I should have learned to rope and ride. Wearing my six-shooter, riding my pony on a cattle drive. Stealing a young girls heart, just like Gene and Roy. Singing those camp fire songs, oh, I should have been a cowboy!"