Not sure for how long your DB2 systems freezes, or what your buffer pool sizes are, so it's hard to analyze the problem. You might consider going to a later Fixpak. Fixpak 9 is fairly stable, but 10a is the current one I believe. At least look at the APARS for fixpaks 8-10a.
There is an IBM Redbook available on the IBM website called "DB2 UDB V7.1 Performance Tuning Guide." In particular, I would look at using the Performance Wizard from the Control Center to allocate bufferpools and other DB2 performance parameters. Also, make sure your temporary tablespace is large enough, at least as large as the largest table, and preferably several time as large if you have multiple simultaneous users.