I was able to run huge queries through SQL*Plus from both my computer and our SQL Server box. This rules out a network issue and an issue with Oracle. The issue appears to be coming from SQL Server and the linked server. I've tried deleting and recreating the linked server, messing with the provider settings, restarting the service, everything I can think or find through Google. Nothing seems to be working, but I have narrowed it down to a SQL Server linked server issue.