Minecarts enter within themselfs and starts moving indepently

When testing with minecarts being pushed by minecarts with furnace, tests runs well but at some point of the test, minecarts entered inside of each others and start moving indepently of the minecart with furnace and much more fast. It lags a lot when these carts are joined. Seems to work with all types of minecarts since they're coded with the same engine.