Also on Ubuntu 18.04 i7-4790 with Intel Integrated Graphic Card.
I think this is not related to graphic card.
If you still want to use transparency from system theme, you can try following:
1. Uncheck "Use transparency from system theme"
2. "Check Use tranparent background"
3. Re-check "Use transparency from system theme"
And the background will become stable with system theme transparency.
I guess gnome terminal sometimes mistakenly checking the grayed out "Use transparent background" setting and disabled transparency, thus cause the flickering.
Also on Ubuntu 18.04 i7-4790 with Intel Integrated Graphic Card.
I think this is not related to graphic card.
If you still want to use transparency from system theme, you can try following:
1. Uncheck "Use transparency from system theme"
2. "Check Use tranparent background"
3. Re-check "Use transparency from system theme"
And the background will become stable with system theme transparency.
I guess gnome terminal sometimes mistakenly checking the grayed out "Use transparent background" setting and disabled transparency, thus cause the flickering.