Engine brake problems?

Hello everybody.
I have a problem, some of you may help me with.

When driving constant in like 40 or 50 km/h, it feels like the engine brake gets in and out. So the speed is not constant.

What can be the problem?

I have upgraded the clutch to a CVTec, and installed a new belt. I also instaled a new helix and spring (OEM), because the old one was worn out.

Could it be engine brake or the brakes? There are no sound of squeeking brakes, so I find it hard to belive it is the brakes.

Tanks for all answers.
:th_smiliepullhair::th_smiliefrustrated