Essentially there are two keys you encrypt with, one is for the server packets to send to the client and the other is to decrypt the client packets the server receives.
These keys are changed each time a game server/client packet is encrypted or decrypted. If done incorrectly it will create a cascading effect where-by the offset is not correct and that means the key is then modified differently and ultimately the packet data becomes jumbled as you've experienced.
You mentioned moving around when it happens which is a possible indicator that your array offset used in the decryption function is short lived. This is purely a guess based on my experience with rebuilding the encryption in my own project. There is an array offset that will start at 2 and a position that will start at 0. as you encrypt and decrypt the packet data, dependent on the size, the offset will increase and decrease.
Motion type packets are 20-40 bytes offset but some packets such as SM_INVENTORY_INFO or whatever its called can be upwards of 2-4k+ bytes offset which is more than enough to throw off your decryption and will cause this type of problem. This is an educated guess, without seeing your code I can't tell you what it is definitively. I'd say go check the offset first, if it's static, it won't work.
If you want further help, post some code snippets of the decryption function, maybe I can spot something obvious.
