My SNES keeps erasing my DK-Country 3 saves [duplicate]

So, SNES Donkey Kong Country 3 keeps deleting my saves. First we noticed that the game has erased one of the earlier saves (game is my cousins, I'm just borrowing it) and one night when we were about to try again the second to last stage the game had erased our save. Well, we started again and got as far as before with no problems in saving. My friend finally got through it and I was so happy that I accidentally ripped my controller off SNES. Of course SNES didn't response and we had no option but to restart. And again the save had been deleted. This has happened few times before, when the controller comes off, the save erases itself also.. The weird thing is that there is one of my cousins saves that have survived through all this so I think it can't be about the batteries... I have a PAL version. I don't know if it matters.

Does anyone have the same problem and/or know how to fix it? Or at least where's the problem? My SNES doesn't do this with any other game, just this one.

When the cartridges of the 16bit console generation were made, Flash memory which can contain data without being powered wasn't as affordable as it is today. Game cartridges which are able to store savegames used memory chips which required a continuous power supply by a battery to maintain their state while not plugged into a powered-on console.

After all the years, the battery in your DKC3 cartridge is likely nearing the end of its lifetime which results in sporadic data-loss. To get it to work more reliable again, you will have to replace it. To do this, you will have to open the cartridge, unsolder the old battery from the circuit board and replace it with a new BR-2032 button-cell battery. Having to use a soldering iron might be a high psychological barrier, but when you actually got one and made yourself familiar with the basic handling, it is a quite easy procedure.

I found a video tutorial which describes the procedure:

Here is a text-based guide with pictures which show more detail: http://motherboard.vice.com/blog/pictures-how-to-replace-an-snes-cartridge-save-game-battery