Q: Fan runs high speed when starting up and never stops, the system is also lagging a lot

I bought this new Imac  (late 2014 5K 27 inches) one month ago because my early 2011 macbook pro has the video issue again and again. Got the logic board replaced twice and it went even worse (now it freezes all the time) and I haven't had a chance to repair it again.

And now the Imac already starts to have some problem. The fan runs high speed when I start up the Imac and never stops, and the system is also lagging a lot. I tried to rest the SMC but it wouldn't help. CPU % and temperature seems to be really low.

Anyone could help?

Thanks a lot