I have the Astra 3400 and a DP Power Mac G4. When I installed the 3.73 update, I had some major problems with the scanner, similar to what you described. I ended up clean-installing my system folder and loading the original 3.7 driver and everything works fine now. Haven't tried fiddling with the 3.73 driver again--I don't think it's worth the hassle. In my opinion, I'd stay away from it.
Hope this helps you out.
-Michael