In SharePoint applications with large customizations using custom code if objects using umanaged resources are not disposed, it would lead to memory leaks. The w3p process memory would drain quickly as more requests keep coming over a period of time.
You could check this quickly by looking at Virtual Memory in task manager. Other way would be to look at Private Bytes and .Net Memory performance counters.
Check your code for few obvious ones:
a) using (SPWeb web = new SPWeb(.....) )
b) using (SPWeb web = site.openWeb() )
c) Similary disposing the SpSite objects
Some other one are:
a) using (SPWeb web = site.openWeb() )
SPWeb parentWeb = web.parentWeb;
and even the
Hope this helps...