External (thunderbolt) display not working at all when in clamshell mode

I have a brand new macbook pro (2014). With a thunderbolt display and external keyboard/mouse attached, I use the computer as a 'desktop' all the time (lid closed). Always worked fine, until I upgraded to Yosemite. Now, either the machine doesn't boot at all, OR (even more strangely), it takes many minutes to boot up, and the animation of fades and also the mouse cursor jump around extremely slowly--the interface is completely unusable and I have to open the display and manually force a shutdown by holding the power button down. Without an external monitor attached, everything seems to work just fine.

Has anyone else experienced this, and has anyone found a work around?

Thank you,

Frustrated with Apple's apparent software quality decline,

Mark