The keyboard of the Clip Zip is specially mapped by software, depending upn the mode currently selected.
I have tried the steps you mention here, pausing / lock / unlock then trying the pause / play again. I could not get the device to misbehave, at least on this one in hand.
In short, how about we try a little diagnostic trick. This doesn’t require a special test mode to do:
We need to see if there is a hardware issue first. Turn on your Zip, and hold the Home button for a moment, until you are at the main menu level. This is the Music > Books > Sport > etc level. Bring it over to Settings, then tap the center button to open the menu. Note that the top choice will be highlighted.
Using the main square 4-way button, press the bottom one a few times to bring the highlight down a few choices. Now try the same thing, but this time use the top button of the quad (with the play / pause logo), to go back up a few lines. Keep trying to go up, then down. Does the highlight roll up and down every time you press the upper and lower buttons? If so, this is great, the switches are mechanically OK.
The oslution, in your case, is to reinstall the firmware in your device. If your firmware is an older version than the current one, you can use the Sansa Updater utility to do this automatically, as described in the firmware link I’ll list in a moment. Otherwise, simply reinstall the latest version manually as described in the linked page. Installing manually has only one tricky step, and that is that the firmware is in a zipped or compressed form. You must extract, or unzip, the file first. In Windows 7, you’ll see an extract button that appears at the top of the screen in Windows Explorer. In Windows XP, rightclickon the downloaded file and select Unzip.
Here’s the firmware thread for you (for the clip Zip).
Let me know how it goes, or if you have any difficulties installing teh firmware. I can walk you through the process.