MS Wireless Comfort Keyoard problem

I have a problem with my MS Wireless Comfort Keyboard. I am using Windows XP
Home.

I am left handed, and frequently use the right shift key so that I can type
in upper case characters with my left hand.

The problem is that it frequently reverts to lower case after typing in only
a few characters. Sometimes it stays in upper case for 10, 20 or 30
characters, but it may revert to lower case after only one or two. I have
double checked that I have been maintaining firm pressure on the Shift key.

This is most annoying, and I am constantly having to make corrections
because of this.

I have new alkaline batteries in the keyboard, and have re-installed the
driver software.

..Her is an example of the problem .........SSSDDeefffddderrrrr
GGHHHJJYYTTRRReedfdss FSssf

Any suggestions would be appreciated.

VG