Debug on 5.1.0.1

Hi All,
I am on 5.1.0.1 with all updates.

I debug a program as interactive, it works fine to switch to the debugging
perspective, source opens, I assign breakpoints, monitor fields, etc.

Now the problem:
When I click on the scroll bar on the right side to go down the source
further, it pages ok for a while, maybe 4 or 5 times, then it hiccups and
starts back about 200-300 lines earlier in the code.  Really strange.  Same
with the arrow buttons on the top and bottom of that same scroll bar.  It
goes down one line at a time and then eventually, jumps to a new place in
the source code unrelated to where I was.  When I use the arrow keys and
page up and down keys on the keyboard, it works fine.

Am I loosing my mind here?  What is going on?

TIA
Duie