Burn/Dodge Tool Problem

I just installed a new monitor and graphics card on my Windows machine using CS5.
Everything seems OK , except when I use the Dodge/Burn tool and try to make the circle larger, after size 150 the circle starts to disappear and by 1100 it's completly gone. I can still burn and dodge but I cannot tell where exactly my tool is working.
I have the NEC PA241W monitor and a GeForce 9500GT card. I have the resolution set at 1920 x 1200 which is what NEC recommends.
Any ideas?

whats the document size?

The problem seems to occur at any image size-
15 x 21 # 72
3.5 x 5 @ 300
9.5 x 14 @ 300

Check your document size settings

The document size seems OK and there looks to be no way to change it.

>image>canvas size

Changing the Canvas size didn't affect the tool problem.
I checked again and it seems any tool that has a circle will be affected by the problem I've described.
I'm having the IT person change out the graphics card to see if this helps.

The easiest solution was the one I overlooked!
A friend suggested checking the version of the graphics card. I updated it and everything looks fine.

you can try
">image>canvas size"

All set ......closing to deter spam