RE: XPS 15Z(L511z) sleep-mode problem after Windows 8 upgrade
Oh no JOK3RAS, the problem occurs(sometimes) also when plugged in. After much research I realized that the problem is related to the drivers. Even if the drivers updated for Win8 are present on Dell site, these are not really ready yet and perhaps it may never be ready(latest date back to 2012).
Win8 had changes for the sleep-mode(S3-S4 levels) compared to windows 7.
Regarding to your suggestion, this is a regular procedure and works well also for me
My problem is that the laptop immediately wakes from sleep(sometimes) at the same time I close the lid. Many others have this problem also with different machines.