Intermittent Error 503 with O365 and ADFS

[Jambo_C]

I have O365 setup with an ADFS proxy and I have an issue where I sometimes get an 503 Service unavailable error when users try to access the login page. However, this isn't affecting everyone, I can be sat at one PC getting the error yet the PC right next to it will load the logon screen perfectly fine. I just had a student attempting to get on, I told him to log onto the next PC and it worked fine on there, this leads me to believe that it's the PC itself, yet when I just logged on to that PC myself it worked fine for me! I had a user yesterday who was getting the error on their whiteboard machine, whilst looking at it I successfully got to the logon screen on my phone and the class of students in the IT suite next door were happily using it! It's starting to really annoy me as I can't see anything obvious, nothing seems to point to it being either the PC or the user and there doesn't seem to be any pattern to it. Any thoughts would be appreciated. Cheers!