Yes -- and thanks for the informed input. I'd done that already, checking the filesystem that is, based on the prior posts. No errors found that I could see. I will keep your additional advice in mind (cache, database, etc.) if I have more trouble.
I managed to get the Clip Zip responsive by doing the 20+ second hold-down on the power button then repowering into the Sansa firmware. After that I removed Rockbox and reinstalled the original (1.01.18A) firmware.
I have since reinstalled Rockbox and I'll give it a try again for a few days. I am using a few themes, custom settings for scrolling, etc., so if I have to make a third attempt, I will forgo these niceties and "keep it simple." But for now it's working OK.
I might as well mention one other little thing. Prior to installing Rockbox the first time, I used Sandisk's utility and update the firmware from 1.01.18A to 1.01.20A, the currently latest Sandisk firmware. However when I installed the bootloader, I pointed it to 1.01.18A's .bin file for patching. I wonder if using 1.01.18A for this purpose, while overlaying 1.01.20A (installed) could have been a factor.