When I get past around 30 minutes of video with some subtitles, transitions, photos, voice overs and music, Studio gets VERY unstable, slow, buggy and occasionally just shuts down. When I pull up my CPU usage at these times it is maxed out at 98-100%.
So up to that point all is actually in control and workable?
If yes then due the 100% CPU usage thing my bet would be on a heat problem ...........................
Get Coretemp and report back with your system's CPU core temperature values when you approach the 30 minute mark on a typical project.
FWIW - I see around 55C on my machine at around 90%+ CPU usage in a well ventilated case and with ambient around 25C (I would say from experience you definitely don't want anything above about 65C ) ...................... and if the on-board graphics can cope well with the first 30 minutes of a project without puking I see no immediate reason to replace that without a lot more detailed troubleshooting and investigation