hyper v 2012 nic adapter

Hi

i installed server2012 on server dell r620 r420 t320

im used with hyper v service

and run server2008r2 on hyper-v

i have 4 adapter on server r620

i config 1 of the nic to use on vhd

the problem is  some time the traffic is very low ( ping very poor)

to fix it i need to remove nic form the 2012 and to configur again ....

im update all the driver and the frimware but not help !!!!

thx

dror