ST7920 lcd screen and hc06 bluetooth module : display issue

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Hello everyone,

I'd like some help please.
I'm trying to use in the same project a ST7920 LCD screen and a hc06 bluetooth module.
I use the screen in serial mode and i initialized it like that : U8GLIB_ST7920_128X64_4X u8g(13,11,10);

Here is the problem, the screen works fine but when i hook up the VCC of the hc06 module (3.3V),
the screen starts to blink and after less than a minute, his intensity goes down.

I thought may be it was due to the fact that both of them works in serial mode but the problem is the same when i don't hook up TX and RX of the bluetooth module.

That's weird because a few weeks ago, everything worked fine but i changed a lot of things in my program. Maybe that's because i changed U8GLIB_ST7920_128X64_4X u8g(10); to U8GLIB_ST7920_128X64_4X u8g(13,11,10)?

HC 06 =>
Serial.begin(9600);
while(Serial.available()){
commande+=char(Serial.read());//store string from serial command
}

I use Arduino MEGA.

Is there anyone who has a solution?
Thanks
Kikiz