After I made a repro from a pirate board here I found that it gives the same controller problem in this clone.
I also got AVE's F-15 City War and it gives me the same problem.
So now I have 3 games that gives the same problem in this clone.
Interestingly AVE's Impossible Mission-II, wich curiously arrived at my home wrapped on a package wich had big red letters saying "this product causes cancer"

Except for my pirate Gradius wich the ROMs are epoxy blobs, could this problem with the joystick be something related to what the mapper is doing + the speed of the PRG ROM?
I asked some people to test these games on this same clone here on a Brazilian forum and it worked fine.
So the problem seems to be specifically with mine.
Any ideas of what I could do to fix this clone once and for all?
Again, thanks in advance.