First of all install the drivers for amd.com also do a clean uninstall.
1_Download and install driver sweeper from here ( http://www.guru3d.com/category/driversweeper/)
2_Download but don't install(yet) latest VGA driver from www.amd.com
3_Uninstall your current VGA driver from add/remove programs and restart and boot to safe mode
4_In safe mode open driver sweeper and click on ATI and then click on Clean and restart and boot normally to windows
5_Install the driver which you downloaded and restart
Sounds like a driver problem to me, assuming that you left all the settings exactly the same for the 3850 benchmark run as the 4890 benchmark run. Anyway, it would be best to do a clean format of your system just to be sure. Those older Catalyst drivers you probably first started using with your 3850 can sometimes leave crap behind. What you can do for now is run drive cleaner pro and see if that doesn't fix your problem. You can google "Drive cleaner Pro" and follow the instructions on the website. You could also use drivesweeper and just follow the instructions Maziar gives.