Probably a contact issue, try one of teh paper tricks.

 

At a last resort, i'd open up the shell and reseat the PCB. At the same time, give the contacts a good clean.

 

Sorry, forgot it was an older cart. Yes, the clips hold the cart together. PCB = Printed Circuit Board, there's a bunch of images http://www.akaio.net/forum/viewtopic.php?t=410&f=8 Acekard 2 is at the bottom. You can use something alcohol based to clean it.

 

What I mean i sthe PCB have moved slightly inthe shell, remove it and replace it. Making sure it's in correctly i.e reseating it in the shell. It probably won't make any difference, but it's worth a go.

 

If it's glued in... I'm not sure if you want to remove it.
Since it has worked for, as you said, several years chances are the position isn't the problem.

I'd say it's more a problem with the contacts... either they're dirty or worn.
What I would do is, take a Cotton Swab (called Q-Tip where I'm from) in some rubbing alcohol and use one end gently 'polish' the contacts on the AceKard to get any dust/dirt/etc off the contacts.
Then use the other end, kind of heavy with alcohol but not dripping a lot, to dab the contacts inside the DS. You want to get the alcohol to almost drip into the contacts getting them a little wet.
Now put in the AceKard and pop it out. Do this a few times but DO NOT TURN THE DS ON!
Give the AceKard another quick wipe if you'd like, it can't hurt.
Eject the cart and let it and the DS sit for AT LEAST 15~20 minutes. Preferably more... say an hour, or overnight to be absolutely sure everything is dried.