Drop FPS, lag and freeze in game

When I start a world, everything is normal, nothing happen, but when I walk around my house and even go farther the game start freeze/lag, sometime FPS drop to 0. After that everything turn to normal. This happen over and over. Sometime make my game crash really bad.
My system is not too good but I think it's not bad that make my game crash or freeze/lag like that
My phone: Qualcomm Snapdragon 435 1,4GHz, Core 8, Ram 2Gb, Adreno 505