See what you can clock them to in 10mhz increments with out the voltage bump first.
My guess is the power target is being reached and the card is throttling back.
 
Fan speed, memory clock, core voltage increase and core clock increase all raise power usage which in turn gets you to power target limit and in turn the card will down clock.

Its not that your system cant handle its that your card has its own power limit built in hence the power target slider. Its Nvidias way of making it impossible for you to cook the card...unless you use a 3rd party bios in which case the power target can be opened up enough to do anything you want.
 
To see if you are hitting any limits you need to scroll down the chart recorder built into precision/ afterburner whichever you use and you will see a section that says power limit.... voltage limit...and temp limit if any of those are producing a 1 on the chart then that is the limit/limits that are causing the card to down clock.