SS2Z06

I have a 2006 sl55 and have been having belt issues for quite some time. I have the 180mm pulley, new tensioner and idler. I originally had a 172mm pulley and was losing the belt quite frequently on full throttle. When I swapped to the 180 all was ok for about 10k miles. I was playing with the car on the freeway when the belt came off again(shredded). All is aligned ok. I'm not sure how to resolve this other than to get a better tensioner system. I am looking for suggestions or companies that make something like this. I know vrp does but $1200 is kind of rediculous. However if it is my only option I will have to go with it. Please help!
TIA-
matt