increase udp_ttl in aix to avoid timout.But still?

[PM Wong]

We recently find some sites couldn't be resolved by our aix
dns server (and the other tru64 unix dns server also).
Finding postings on various newsgroups suggest the default
value for udp_ttl on these 2 OS are 30, which could be low.
I increased them on Tru64 Unix and those sites which could
not be resolved previously are then okay.
But doing the same increase on aix 4.2.1 still couldn't 
resolve some "stubborn" sites (though overall there are
less unresolved sites).
What can i do further ?
Change the named.ca (the cache servers' file) or find
the relevant factors system related to aix instead ?
any suggestions