i use 10 foot long cables and don't get that issue with my vg248qe monitors
could be a problem with the cable, monitor, graphics card, (highly doubtfully) pci-e slot on the motherboard...
you'll have to tediously go through all potentially malfunctioning components to figure it out. no easy answer for you there.
what's your monitor's make & model? check if there might be a batch of malfunctioning monitors within a spectrum of serial numbers.
what's your gpu's make & model? try different drivers if nothing comes up about a potentially bad batch.
worst case, you might need to RMA something.
if you're adventurous and out of other ideas, you could get toastyx's custom resolution utility and make custom 144hz resolutions using reduced timings to identify the bottleneck.
overclock.net and hardforum are usually on top of issues of this sort. consult those sites for further help.