27" imac to external display vga or hdmi not detecting

I have a mid 2010 27" imac, with i3 processor, 16 GB ram, 1 TB hdd. Have both a mini dp to vga and mini dp to hdmi adapters. samsung 23" monitor with both hdmi and vga inputs. hdmi adapter never worked, the vga adapter works in a highly intermittant fashion. Did a fresh install of mountain lion, no difference. Very frustrating. Have also reset pram and smc. Anyone have any thoughts?