[Lilux-help] DNS question

Hi, I have a (possibly stupid) question about DNS and bind. Let's suppose I have a network with DMZ, with external and internal DNS servers, with different definitions for my domain in both (MX is different when seen from outside than from inside, more stuff on the inside, both configured as masters). Now, for whatever reason, my internal servers are slower to resolve external addresses than the external ones, and I'd like to use those as forwarders, for stuff other than my own domain (and subdomains), instead of going directly to the root servers. When I try to go through the "forward first" option, it seems that *all* requests go to the external DNS servers, which is definitely not wanted - I want the internal servers to continue serving requests for my own domain or subdomains. I didn't as yet find the correct way to implement this (maybe still not enough coffee or I didn't hit upon the right manual/ howto/whatever - checked google, ISC, tldp, ...). Could someone point me the right way, if it's doable at all? TIA, Eric