Well i have came across a wierd one today. We are running 7.5 with 2 projects running. We are having issues with the clients not reading OPC data when they are started up. You can let it set for several minutes and sometimes it will start reading values. The thing is a designer running on the same machine has no issues with reading OPC tag values during this same time. We are seeing this behavior with both projects and on every client dosent matter which machine. We reset the gatway yesterday evening and all was working like we would expect but came in this morning and we were back to having the client read errors.
Note that the overlay on the components are a green ? if that gives you anymore info.