Thunderbolt display gradually stopped working for external monitor.

I have an external monitor connected to a 2013 iMac through a thunderbolt adapter. The monitor began losing signal randomly as if the computer was going to sleep. At first I could get it working simply by plugging the adapter into the other thunderbolt port but throughout the day the length of time it was working for grew shorter and shorter. Now when you plug it in to the port the monitor just briefly flashes 'No Signal' and goes back to sleep. I have tried resetting NVRAM and SMC several times. The monitor and adapter work when connected to other identical iMacs and when I try other monitors and adapters on my iMac they also get no signal from the Thunderbolt port. Any help would be great!