Radio transmission is very poor

I am very disappointed about this bike computer without cables. If it works after setting off sometime, he turns again sporadically and switches also sporadically again, but then one a few kilometers to. As if he had something like a Power Save in the display. The Verkäüfer sent me equal to SIGMA, he or she can not do anything. The Fa. Sigma Says that it is external radio interference, in which my bicycle lamp and all environmental influences are to blame. And in a digital transmission! Under a high-voltage line of bicycle computers but sometimes works :-). Our son has a cycle computer also by SIGMA with cable and he has no problems.