2nd monitor kills my cpu

Hello,

anyone have or had this problem ?
when I use my 2nd monitor - via thunderbolt, with a macbook pro core i7 -
after a while and with a not too heavy patch (around 33 to 50% - quite stable)
the cpu goes to 80, with clicks etc...it return to normal as soon as
I remove the thunderbolt plug

???

thanks

Eryck