Actually, it sounds like you may have a firmware issue.
If the player is constantly restarting, you’re not going to be able to get a reliable connection to your computer in order to format.
Here’s what I’d try:
If you don’t already have it, download and install the Sansa Firmware Updater:
2. When the Installation gets to the final screen with the FINISH button and tells you to connect your player, put the player into recovery mode and connect it.
-----How to put player into recovery mode: Make sure player is powered off completely --> Push the HOLD switch (on top of player by the headphone jack) over so you see orange --> Press and HOLD the record button on the side of the player --> while holding the record button, press the power button once (do NOT let go of the REC button) --> if you’ve done this correctly a message should show up on your player saying “Welcome to Recovery mode, connect USB” or something along those lines. At this point, connect your player to the computer (you can let go of the record button, btw!)
3. Once the player is connected, press FINISH on the installation. In a few moments a white box with a red S in it should appear by where your time is at the bottom of your computer screen. Right click on it and select “check for updates now” and all the program to run. You should get a message within a few minutes (depending on your internet connection speed) telling you if there were any updates found.
4. If updates are found, follow the onscreen instructions and install them.
If you already have the Updater installed on your computer, just skip the part about the installation. When you get your player into recovery mode and connect it, the Updater will automatically start within a few minutes if everything is working correctly.
If the Updater determines that there were no updates for your player (meaning you have the most recent firmware), you will need to contact Tech Support (1-866-SanDisk), explain what’s happened and what you’ve done and they’ll have to send you a manual update and possibly a format.