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Some more info please:
What are your computer specs? Please include total RAM and VRAM.
Have you installed Optifine, this has been known to cause issues, remove it if you have.
Have you tried lowering graphics from fancy to fast? Also what render distance have you set?
Also are you running the latest version of Java? It does sound like a garbage collection issue because restarting the client fixes it.
I'm working shift at the moment so might not response that quickly.