U3014 manifesting errounous behaviours using 3m cable

Hallo,

I have a Dell screen u3014 that manifests some  errouneous behaviours when it is connected to the PC via a DisplayPort cable of 3m. I noticed that the problems disappear if I use a shorter cable (2m) DisplayPort cable. I am pretty sure that the problem is related to the cable because I tested it with different computers and different graphic cards. So the problem could be the cable itself (maybe it is broken), or maybe the problem is that is too long (3m). Do you know if the length of the cable can be a problem whem connecting it to a dell screen ? THanks