We had a similar problem where certain websites were coming up slow or not coming up at all (yahoo.com in particular) recently. We have 501 too at this office.
The first thing you should make sure is that your DNS and DNS forwarding is working correctly. See if you can ping via name on your internal network. If you can, then check via nslookup to see if you can get DNS lookup's for outside names such as yahoo.com.
Assuming you can not, then change the server to your DNS forwarder and see if the lookups work.
Assuming your DNS forwarder is working but you aren't, then you can probably guess that the PIX is stopping it somehow.
You will have to go through a trouble shooting routine. The best way is to pick a low/no traffic time. Log on to your firewall. Try a website via name outside your firewall. Look at the log on the pix and see what denyed it. Make the appropriate changes. In our case, we had a max packet length on the DNS response of 512 which we had to change to 1024 and everything started working fine. As to why, DNS responses were suddenly bigger than 512, I have no clue
