I think this is a known issue with the 2400s, so I can shed some light on this:
The original design of the 2400 used tin only contacts as part of the power supply board. It turned out that these contacts (on one connector in particular!) were susceptible to fretting, a combination of wear and oxidation in this case. If it got bad enough, the fretting would cause an intermittent connection to the rest of the instrument, which results in the instrument randomly turning off and shortly coming back on. There was a revision to the 2400s to change that contact to gold which prevents this problem, so you must have one of the earlier 2400s. You can send it in to be repaired, or, if you want to try fixing it yourself, you can follow these unofficial, undocumented, warranty-voiding directions:
Lightly cleaning the problem connector is likely all that's needed to make a good connection again. I wouldn't recommend sand paper as that may destroy the tin layer, but a soft cloth or a paper towel and a little bit of force will probably be enough to remove any built up oxide. Getting to the connector is a little tricky though: it's at the bottom of the instrument so you will probably need to remove the analog board that's immediately visible when you open the box.
To identify the problem child connector follow the power from the line voltage input connector, where you plug into the box. There's a wire pair with brown and blue wires that carry the line voltage and take it immediately into a board, next to where the power switch actually controls the instrument's power. Right next to that, the same wires come out of that board and carry the line voltage over to the internal power supply that creates the SMU's rail voltages. On the far end of that board (directly behind the SMU's front panel) you'll see two connectors with black, red, and orange wires. One set goes to the fan (which is fine) but the other set goes to a different board on the instrument (see the photo I added), that's the one with the problem. You would need to carefully unseat that connector (being careful not to pull the wires from the connector, they're just held in with crushed metal) clean the tin contacts on both the male and female side (maybe a very small wire cleaner would work for the female connector?) and then put in the instrument back together.
I'm not going to say I fully endorse this method (I don't think I can since I told you to open the box), but that should keep you from seeing the intermittent power supply problem.
Edit: I found an open 2400 so I fixed some descriptions and added a picture with an arrow at the bad connector, the one I found is a newer revision with gold contacts. Also I'll stress that removing the top analog board is not exactly trivial, so if you're not confidant in your teardown skills, better to send it in for repair.