U2312HM and 2011 imac

I purchased a Dell U2312HM that has a displayport input. I hooked it up to my imac and it outputs, though there is this flickering of sorts happening. At first i thought it might be the cheap cable I bought from monoprice (MDP-> DP), but when I tried it with my MBA there was no flickering whatsoever.

I’m thinking my thunderbolt port on the imac is bad.

Any thoughts?

Just bought a MDP>DVI adaptor and no problems.