I highly doubt that the master link by itself is causing your problems.
Can you post pictures?
If it is loose only in one spot, it could be that the chain links are too stiff in one area of the chain to let it flex properly, or it could be that the rear sprocket is not centered on the wheel.
Also in some cases the sprocket teeth on the engine and/ or rear wheel have a bit of manufacturing burrs on them and this will cause the chain to ride up on the tips of the sprocket teeth instead of down in the valleys like it should.
Carefully watch the chain as it travels around both sprockets to see if it is doing this.
You will have to remove the engine sprocket cover to do this on that sprocket.
If it is happening, use a file and file down the teeth on the sides only, not in the valleys to very slightly thin the teeth so the chain will slip onto the teeth easier. Usually you do not need to do the whole tooth length, just the outer ends.