This certainly doesn't sound right. Getting hot is expected, but it should never keep switching on and off. What voltage of pack are you charging (6 cells I assume)? It seems to be acting like you're overstressing the internal PSU, but it should limit charge current before that happens...so perhaps you have a defective unit.
If it's only started happening recently, then I'd imagine something has failed inside the charger. Might be warranty time.
