Scroll wheel is ‘choppy’ in Windows Live Mail

I'm running WLM 2011 under Win-Vista and recently noticed program operation I'd not seen before.  Within an e-mail message, whether it's plain text or HTML with (or without) graphics, I can grab the bar to the right of the pane and smoothly scroll up and down through the message.  But when I use the scroll wheel on my mouse, scrolling is very intermittient, 'choppy' and often lags; that is, it continues to jump around a bit after I quit rolling the wheel.  Scroll-wheel scrolling is fine in all other programs and, in fact, is even smooth in the main Inbox pane of WLM.  But within any individual mail message operation is quite ragged.  I have run the Windows Essentials repair utility, which didn't help at all.  Anyone else notice this?  Thanks.