Every now and then I have problems loading chunks when I am on a slimeblock piston mechanic (such as a plane, rocket) which causes my minecraft to freeze for about 1-3 minutes, and I am normally using 15% of 1019 MB of allocated RAM, so I doubt it has anything to do with a lack of memory. I have no idea what causes this to happen but I know that I'm not the only one that has this problem, since a friend of mine reported the exact same issue.
It might have to do with certain entities loaded in certain chunks, but again. I have no clue what sets this problem off