HDMI Cable Problems

i have a new htpc computer that i built. it has the ga-ma780g-ud3h gigabyte motherboard with hdmi output. i have an lg 42" lcd hdtv with hdmi input. i bought a somewhat cheap hdmi cable fromt target for $20 or so. it worked when i plugged in the cable at first but if i bump the connection (i have only bumped it at the motherboard) the signal goes out and i have to do some fiddling around with the rgb output from the motherboard to the tv in order to get the hdmi to work. i think there is something wrong with the cable since if i move the cable even a foot down the cable from the motherboard it will still go out. do cables really make a difference? also, is this thread in the right place? i wasn't exactly sure. thanks in advance for help. this is really annoying me quite a bit.