What other suggestions are you looking for? Currently you have a linear problem with only one solution. That is making it stop rebooting over and over.

There is only one solution to that. Remove the battery. Don't turn it back on. The N900 behaves differently if you boot it up while attached to a USB cable then it does without it.

So take the battery out. Put it back in. Leave it off. Attach it to your PC via USB, then power it on again. Also the firmware updating software works in the same method.