The game had two problems.
First one was the music that sometimes when switching on the game was glitched (very fast tempo or corrupted voices)
The first problem was fixed by replacing capacitor C7 which was used to reset the chips and didn’t work correctly.
Second problem was some black pixels on some elements of the background.
After shorting some data lines of the gfx maskroms I found the one responsibile for the background images.
Normally when a markrom fails you get blocky graphics. In this case was only some bad pixels.
Looking on the solder side of the pcb I immediately noticed one pin of one address line of the background image markrom touching the network resistance beside which forced the address line to 1.
Disconnecting it solved the problem