Why do I need to keep reloading my chunks in minecraft?

My fps is fine, up until a point where I need to reload my chunks! I need to press f3+a to get better frames.

I could be getting around 90 frames, then gradually they'll drop to around 30 and it'll be unplayable. So i'll press f3+a but it'll go back up to 90, then back down gradually.

I'd have no problem about this as it's no burden, but it's cheating on servers like MCSG and Reddit UHCs because you can see people when you press f3+a for a second. I don't want to cheat, but if I see someone I can't just let them go.

Anyway can someone tell me WHY i need to keep pressing f3+a to keep my frames, and how I can stop it!

Download optifine, it helps. Alot of people have this problem.