Windows 7 Ultimate x64
AMD 965 Black Edition
AMD Radeon 7870 2GB
Java 6, 7 and 8 (tried them all)
I have the same problem. It all started with snapshot 14w10. It's basically what people (I think) describe as a memory leak in other games. I can play about 20-30min tops and my fps plumps down from 40-50fps to 13-19fps tops.
I have adjusted graphics settings, performance, anything you can adjust. Even tried Java 6 to 8. With resourcepack, without resourcepack. I literally tried everything that came to mind and the issue continues.
However I have the feeling not everyone's got the same issue. A friend of mine, no gaming PC (rather an older model) doesn't seem to have any major issues. However this might be due to the fact, that her fps is already quite low and therefore for her it doesn'T make that much a difference anymore.
I am playing on Windows 7 Ultimate x64, AMD 965 Black Edition, 8GB Ram, AMD Radeon 7870 2GB. THere shouldn't be any issue.
Once I start 1.7.5... I have no issue whatsoever even after being in the game for hours.
I hope this helps.
Also had the chance to reinstall my PC and nothing unfortunately helped. So it's definitely not my system, I can rule that one out for sure now.
EDIT: And I forgot, that I have the same problem in singleplayer as in multiplayer.