LCD display glitches with interrupt

[fratello]

I have three "type" of LCD display ; all are HD44780 compatible, all are Made in P.R.C. .
Two of them "react" verry slow : it's impossible to use "scroll effect" and, sometimes, glitches...
One of them (top of picture, left) it's OK ! All the informations are displaying correct and without any glitches, no matter what.
I tested them using same hardware : 16F684 - without ussing interrupts !!!
Maybe the hardware inside of LCD's it's the problem !

[Art]

It does appear to be the LCD, it's a VFD actually, but with the same controller so LCDOUT commands are used.

I am able to confirm everything is fine if I do not update any custom characters, but can use them if they are defined
at the start of the program and then not changed, so no scroll effect

One interesting thing I learned is that if a custom character is being displayed, and then you change it,
the display does not need to be updated, the character changes instantly on the display.