At first (at the start of 2015) the system started freezing after two or more hours of play (inconsistent). When it started to get so bad that I couldn't even play for half an hour, I opened it up and cleaned out the dust for the first time since it went in for an RRoD repair in 10/2008. This seemed to fix the issue for a few weeks, then I had to start using exterior fans to help with cooling...which fixed the issue for a couple of months.
Now the system freezes early on start up, and seems to last longer as it gets warmer, but only lasts about five to ten minutes at most.
Already troubleshooted the harddrive and optical drive. Already cleared cache. I'm thinking I need to re-paste the heat sinks, but would appreciate any other suggestions (other than buying a new system, of course).