I'm also having this problem, and it just started in the last week or two. I've spoken with apple and HP (the manufacturer of my external monitor). Apple was helpful, HP was the worst, but at the end of the day I still have the problem. Forcing the card to nvidia didn't help, nor did these two Apple suggestions:

Reset the System Management Controller (SMC):
- Turn off your system.
- Press Shift-Control-Option-Power.
- Nothing will appear to happen, but the reset was done.
- Restart.

Reset the PRAM:
- Restart your system.
- Before the gray screen appears, hold down Command-Option-R-P. (as in the letters, R and P)
- The gray screen will appear, chime, and then go black as your computer restarts *again*.
- Keep holding the keys down until the 2nd time the gray screen appears and chimes, then you can let go.

None of these solutions worked for me, but they're working for other people so I thought I'd post them.

With my system, I can kill the 2nd monitor just by clicking "detect displays". Reboot, external monitor is fine. Detect displays, monitor loses signal. Like clockwork.

A couple days ago I thought I had it fixed. My external monitor recommends 1366x768 resolution, but I noticed my settings were for 1360x768. I made the change, and was able to go in and out of sleep mode no problem. But that's not working now.

In the interest of helping others as well, I did switch out monitors briefly during my "try everything" blitzkrieg the other day. I'm wondering if that temporary switch helped at all. I'll have to try it again.