thank you all for your nice replies. my question is answered already. i just was wondering if i did something wrong in mach3 or if it could be something with my crappy ebay controller beside of some of my "build my own cnc router design flaws". i am now pretty sure that the problem comes from the machine. this is what i wanted to know. my machine at the moment is work in progress and i know the things which have to be changed. thank you very much so far!
next thing i will do is to change the spindles, couplings,... for the y-axis. at the moment i do drive the y-axis with two spindles and two motors. running the two motors in sync is something that causes troubles all the time so i will change this to work with one motor and two spindles with a belt drive. i also will use professional couplings then. i will see how this works out and if it turns out good, i will then replace the x-axis and last but not least the z-axis.
at the moment i am happy with the machine because it works like it can and i am looking forward to make it better.
have a nice day, all you nice people out there.