Not an Apple Guru. I know windows had a lot of similar problems. Waking from a sleep state has often been an issue with laptops and dual monitor setups. I am going to guess at a few possible ideas.

1. Is your OS patched up fully?

2. Is your BIOS chip updated on your motherboard?

3. Are you running proper drivers for the monitors?

4. Are running the newest video card drivers?

5. Have you played with different configurations in your power saving areas?

6. Have you considered not allowing it to sleep (change your config) and just physically turning off your monitors, and turning them back on when you need them?

The last one is a last resort work around if the top 5 don't work.

Just bouncing a few PC based ideas around, not sure how well they will translate to the Apple World.

Roman