pots were ok, the same for the entire 3D circuit. I had some oxide on the power connector...
This oxide caused another problem, this time permanent. 2 cars glittered in the middle of the screen. This is a movie of the problem
https://www.youtube.com/watch?v=Zavb75Ls7poThen I checked all EPROMS but all of them were ok. Removing Eprom by Erpom I understood that
EPR 1248 is dedicated to violet and green cars
EPR 1247 is dedicated to yellow and blue cars
this is a movie with the EPR1248 removed
http://youtu.be/MAIIqsHF9Usswapping those Eproms, cars swapped positions too but the problem persisted on violet and green cars so I checked again the Eprom but the checksum was ok.
Anyway I burned again the EPR 1248 and the the game returned to play in the right way.