Try to string it full poly with Weiss Cannon. They are notoriously known as a poly that holds tension very well. I like to string it at 54. But with something that rewards string movement for grip like the 99s, you might like to try to string it lower because the tension will maintain its point better with these strings.
Also try Signum Pro, their polys are good too and a lot of people say they are a bit of a stiff poly which might bode well for a racquet that needs more tension maintenance.