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