Screen Jerk on 10.5.6

[Alphonse Mc Clouds]

My External Screen Jerks after update to 10.5.6

but it smooth on 10.5.5

I use EFI String to active graphic card I user Nvidia 8500 gt

 

it will jerks only when i switch off main monitor an play video on external monitor

 

both on plex and Quictime (maybe DVD app)

 

but if I switch on main monitor everything work perfectly

 

Anyone help

 

sorry for my poor English