I just installed my first hacked firmware, that being iXtreme @ 5x speed.
Seems to work fine, except for the following:
In CoD4, for example, during loading of mission 'The Bog', I get a 'clean your disc' error. This seems rather strange, since the disc itself is spotless and it's a quality brand too (Verbatim DVD+R DL). XDVDMulleter confirmed my backup is completely okay.
Is it a firmware problem? Does anyone else with a similar setup experience similar issues?

Were your Verbs made in India or Singapore? If India, it could be bad media but if they were made in Singapore, then it would be a cause for concern.