Monitor turns on and off on multiscreen mode

Hi everybody.

I've a latitude e5450 Intel I5 with graphics Intel 5500.

My laptop is connected with 3 monitors, using a docking station (an old advanced DELL e-replicator).

In 3-screen mode, one monitor, that one connected in HDMI often turns off and on, even now I'm writing. The others are vga and e-port.

I'm quite sure it's not monitor's fault, because in one-screen mode it doesn't happen.

The frequency is the same for all 3 monitors.

Can you help me please?