I'm not familiar with Dell desktops, but on any of my system builds my motherboard always includes a utility to monitor temperature, fan speeds, etc. Sometimes this information is available in the BIOS, along with a setting to specify at what temperature you want the system to shut itself down (for its own safety). This isn't what's happening to you, if the system is restarting and not just shutting down. But there might still be some indicators in there that your system is getting warmer than it likes, and some component (maybe the TB drive) is unable to handle it.
The memory is an easy thing to rule out with memtest. The power supply is also a possible culprit. I always have a spare for testing purposes, but if this is only happening once or twice a month it could be a long, pain-in-the-butt process to isolate the problem.
Adding a new fan is easy, if the OEM motherboard has a spare connector for it. You should have a fan on your CPU and probably a case fan of some sort. Look at the connectors on the motherboard that those fans are plugged into (they're probably labeled CPU FAN and SYS FAN, or something like that). Then look for another on the motherboard labeled similarly (probably SYS FAN2, or the equivalent). If you have another connector like this to power the fan, then you can mount a cheap $5 fan and improve your airflow/cooling. This might not be the problem, but often is, and increasing airflow is NEVER a bad thing.