ATI Radeon HD 5770 mini displayport turning on and off

I am currently running a Mid 2010 Mac Pro, with the latest update of Yosemite. It is a 2x2.4Ghz Quad Core model. The problem i am having is with my mini display ports turning on and off with my Samsung 27inch display SyncMaster S27B350, connected with a Apple Mini-Display to VGA adapter. I also have another second Samsung 23 inch S24D300 attached to the HD 5770 via DVI.

My issue is that for some reason every now and then the 27inch display all of sudden isn't being recognised via mini display and it only recognises the DVI, sometimes it will turn of and then straight away turn back on again. Sometimes it will turn of for ever and need a reboot. Now i have had this problem on one of the two mini display ports and just changed to the other one and thought nothing of it and now it is happening to the other port a few months later.

Any suggestions on fixing the problem?