Minecarts disappearing

I was riding a minecart pushed by a minecart with furnace.(The minecart with furnace stopped several times with no apparent reason and needed to be clicked.) The game became extremely lagged due to the ride (this problem started with the 1.4.2 update), occupying more than 1GB of RAM. I clicked "save and return to main menu", and when I returned, the two minecarts weren't where I was, or in other point of the trail.