Problem with sensibility

[Fran.Raccanello]

Hello, I have a problem with some keys. For example, when I press E3 and G#3 at the same time or the G#3 when E3 is being pressed, the G#3 will sound loud, but really loud, like the louder possible. It happends with severous 3rds mayors all over the Keyboard (A/C3# 3,4 and 6) (E/G# 3,4 and 6) (C/E 3,4 and 6). As far I know, it must be the rubbers but I changed them last year. I am really afraid of open the piano and break something like circuit. But I dont have the money to pay an expert (Argentina is hard). Can someone help me? How do I clean it properly without screwing something? Thanks.