Computer Browsing issue on W2K3/NTdomain

We've adquired a new box for upgrading our platform from a NT4 SP6 domain to a Win2K3 AD. We've configured DNS, Wins and DHCP on new box, and began to move clients from NT domain to Win2K3 AD, but it has arised an issue when a workstation tries to browse through network places (we use Win2K SP4/WinXP SP1), the most of times we get no machines listed on domain. However, they're getting their DHCP assignment, they can access most of resources on other machines across the network (entering remote destination IP\resource name on a Run dialog box), but they still cannot see anything on domain space. We've reviewed some articles here, and got aware about Browstat utility, and made some test in our enviroment. It turned out that our -workstations- had enabled Computer Browsing service, so we disabled it on all the machines. However, since we get the right source for Browsing services (our Win2K3 domain server), it happens that browsing works fine for two mintes or three, then falls and most of the workstations can't browse domains. At this time, we have two domains running: Old NT4 domain (with most of our clients, who haven't been moved yet because of described browsing problem) and new Win2K3 domain with almost 40% of machines active, using (suppossedly!) Win2K3 server as Master Browsing. Can anyone give some advice on what can be happening? It could seem as something is not working ok on the new server, so it stops responding request from browsing clients for a while (a long while) and then it works again for a couple minutes before failing once again... Thanks in advance for your comments.