Primary and secondary screen shake

Before Christmas I upgraded the memory of my MacBook Pro 13-inch, Late 2011 from 4 GB to 8 GB. Since the upgrade, both the primary and secondary display occasionally shake (sideways). It never happened before the upgrade.

OS X is 10.9.5 (haven't dare to upgrade to Yosemite yet). OS X reports 8GB of memory.

Memory is Kingston 1333 MHz DDR3 as per Apple specifications.

System check on boot does not show any problems with the memory modules (both quick test and exhaustive test).

Incidentally, the system sometimes feels as if has not been upgraded, I used Lightroom extensively and often does not feel as it had 4GB more of memory.

Any help/suggestions/thoughts much appreciated.

Edit: Happy to move it the Notebooks community if required.

Message was edited by: Movega