I have actually found that the 3.3.4.1 firmware is the most stable for me but still has compatability problems with some netcards, in particular older Intel wireless cards such as the one that you find in a Compaq nx6310. I've found newer net cards to be much more stable but even so, I shouldn't have to spend hours and hours finding this sort of stuff out. To test I actually installed an old BT 2Wire adsl wireless router and guess what? On the BT router I had no downtime from any wireless in that particular office at all for a week (untill I re-installed the draytek, that is ).
Agreed, Draytek really do need to do something about this as it will be losing them loyal customers.