For now, I think the problem has been solved.
This is what I did. I flashed back to 1.15 (from the 1.12 I flashed when I got the problems initially) just because it is the latest. Now, even while most of the configuration wasn't lost I logged in and made a 'reset to factory defaults'. After that I just started configuring the router little by little until the complete configuration was done.
I think the only problem I have right now is that one client gets DNS resolution problems. I'm not sure if that is the router's fault or not but the stability problem was solved after I did the reset to factory defaults.
To get into the bad stability mode what I did was to upgrade from the original firmware (I think 1.02 _or_ 1.12... not sure anymore) to 1.15 and use the saved config file afterwards.
I may not be able to reproduce the crashes anymore (and that's not a bad thing I think:).
If I can edit the OP title I'll make it read [solved] unless I run into the problem again.
I still have more configurations to restore (subnets to be defined and rules for virtual servers) but I'm keeping my fingers crossed.
Thanks Lycan!
PD. The only thing I'm not too happy about is having to reboot to apply most changes. It pisses people off so they get mad at me and my router, and in return I get mad at them and at the router
