HowTo: Fix a resetting Dreamcast
How to take apart the DC, continued
DISCLAIMER: I, nor anyone here at Consoleia, take any responsibility for any damage done to your console, through this proceedure or any other. Should anything happen, it is entirely your own fault. Proceed with caution. If you have any doubts as to whether you can perform this operation, seek a professional to repair it themselves.
Power Supply Woes: The cause of all life's problems
From here we can proceed to actually fixing the cause of your resetting problem: a poor connection between your Dreamcast's power board and motherboard. On the left of your console (see above for reference), you can see the power board: it's where you plug in the power cable, and is secured to the motherboard by screws, a cable, and a set of pins. Let's get to work.
First of all, you need to remove the cable at the top of the board. It's grey, with a white connector, and has a release catch which you can use to gently pull it from the board. You can then remove the two screws which hold it down onto the motherboard: two small, golden ones. Keep these in a safe spot, like you did with the case screws.
You may now (gently!) lift the power board away from the motherboard. You'll notice some plastic shielding (keep this) and 6 pins underneath: it is these pins that are the source of your problem. Carefully replace the power board down onto these pins, making sure you make a good connection between the two, and ensuring the power board is fully seated upon them.
From here you should replace the screws and cable, then replace the Dreamcast's case, before powering it back up and playing, happily reset-free. Hurrah!