You need to download the FW from the Archos site, preferably onto a PC, copy it onto the Archos by booting the Archos into Recovery mode (in exactly the same way you must have done when installing Uruk in the first place). And then choose to update FW.
This all assumes, of course, that when you installed Uruk 0.7, you then decided to remove the Android FW, using the Recovery Menu option. Otherwise, if you didn't remove the whole Android FW by choosing that option, then the original FW will still be there, and all you have to do is boot into the Recovery Menu as usual (power on while holding the Vol Down button until the white Recoveru screen appears), then select the "Android" option, instead of the "Developer" option.
Then, if you want, you can choose to remove the Developer boot option, and Uruk will be removed with it.
By the way, I've been running Uruk 0.7 as an "Internal" install, continuously for weeks now, without a hitch.