2400 MP Projector Searching for Signal with Just One Laptop

I have a Dell 2400MP projector.

With the one laptop, a Vostro 1520, Win7 Pro SP1, the projector displays searching for signal... at 1024x768 60hz
I have downloaded/installed the latest Intel graphics drivers.
When I plug in the VGA cable the PC recognizes the Dell 2400MP projector and reduces its screen resolution.
But, the projector says searching for signal...
If I reduce the resolution to 800x600 the projector correctly displays the computer screen.
As soon as I return the resolution to 1024x768 the projector returns to searching for signal.

With another Vostro 1520, Win 7 Pro SP1, the projector displays the computer screen correctly at 1024x768
With another Vostro 1320, Win 10 preview, the projector displays the computer screen correctly at 1024x768

I have tried multiple VGA cables with similar results so don't believe it's a cabling issue.

The problem laptop and projector have been in use together for a number of years, this is new behavior.

Help?