new iMac has problem with detecting the 2nd monitor

I just bought a new iMac (yosemite is installed by default).

The new mac has problem with detecting the secondary monitor. So far, the solution is to unplug the thunderbolt cable and plug it back in. Often I need to plug and unplug several times to make it work.

Whenever I do not use this iMac for a while, the 2nd monitor automatically becomes unrecognized. Then I need to do the unplug and plug exercise again to make the iMac detect the 2nd monitor.

There are two thunderbolt slots on this new mac. They both work fine with the ethernet cable.

My previous iMac (manufactured in 2009), which also have yosemite installed now, does not have this problem. I used the same cable and the DVI adaptor.