the hot pixels occur suddently after 7 minutes (before, none is seen), then over time some more pixels grow. (from dark grey to almost white).
Well then, its seems there are two possabilities.
First off, you may have a hardware problem with your camera. A marginal sensor or a sensor not mounted with good heat sink contact perhaps. I guess we will not know if this a problem with the camera until others have reported the same result - or not getting the same result.
Alternatively, its possible you have discovered why Canon limits the time duration in certain video modes. Where you shooting in one of the HD modes? Does CHDK allow you to shoot longer in that mode than CHDK does? What happen after 7 minutes when you are shooting in one of the lower resolution modes? Do you see the same thing when CHDK is not loaded?
Either way, its does not seem to be something CHDK is doing - or anything CHDK can prevent.