We recently had a SRT in with a similar problem, but with stock pulley. After 5 mins of hard driving the SC would be MIA.

We installed a new pump and new HE, courtesy of LET, and problem fixed. The stock HE is pathetic in comparison.

Also, when you replace the coolant use the OEM MB stuff, it's only $20.00 a gallon non-diluted. That's about the same price as the aftermarket stuff.