vxd files are Windows virtual device driver files. They aid in making your hardware run. Intermittent failures of devices that are caused by vxd files are not uncommon in Windows 9x. What happens is that in an attempt to shorten boot time, Windows compresses a lot of commonly used vxd files (video drivers are an example) into one big vxd file called vmm32.vxd. Although this is good in theory, it is not always reliable. Some of the symptoms are crashes, BSOD, and shutdown issues.
A failure in RAM *MAY* be a cause here, but I'd suggest looking at a way to upgrade your vmm32.vxd file. You can download one from here:
http://ftp://ftp.fluidlight.com/pub/nospin_files/VXD_FIX.zipfor Windows 98, 98SE and ME
The instructions are in a readme in the zip file. One thing to note, you will need your Windows install CD around.
As far as the 3 short beeps happening intermittently, I wouldn't look at the memory. It seems to be the motherboard here, as you have swapped out memory you thought was bad only to have the same memory begin to work again.
Hope some of this helps...