This is another one of the reasons why I questioned the use of those pulleys on the other thread. There have been issues with aluminum pulleys since day one, the problem is a lot of you guys don't drive your cars enough to realize how big of a problem it is.
Honestly, as much as some of you don't like the plastic pulleys, they are the best. Many are failing only after 100K miles which makes a lot of you think they are crap. But initially they are lighter and of better quality than the cheap steel pulleys that are bought at numerous places that I won't mention. If your tensioner pulleys do not spin freely - like as in take a few seconds to coast down after being spun up - then they are creating drag and contributing to belt slip. A quality plastic Gates pulley is the best - and from the look of them they appear to be OE manufacture. Unfortunately they are not available (as far as I know) in a wide enough size for the 10 rib belt.