Thunder 1220 Charger

cycles on/off every 55 seconds. will ramp up to 6amps charging at 48V, continues at that rate for 55 seconds then stops charging, drops to 0 amps then ramps back up again. does the same thing when first plugged in or running for an hour strait. been doing this ever since i bought it for over a year now.
my batteries get charged no problem, just takes longer with the constant cycling. i read on an RC forum a person had the exact same issue but never a resolution.
charger is powered by a 750W server power supply.
any ideas?

Could you measure voltage values during the cut-off?

I'm wondering if it is in a thermal cycle. If you put a fan on it, does the cycle time change? Does it behave the same at 5 Amps?