How can I recover damaged camcorder clips

I have a Panasonic HDC-SD60 camcorder, an IMac with OSX 10.6.8 & IMovie 8.0.6.

I SD card I use has worked fine up to now but when I tried to download the latest clips, one clip downloaded maybe 5% of the clip & showed an error & another did maybe half the clip & didn't see anything more.

When I look at the clips on the camcorder, there's a long delay before it plays, I can't scroll & other issues. I know the SD card is the problem because when I put another card in the camcorder, there's no problem.

My guess is that it has something to do with the erasing various older clips to make space a number of times. How do I recover the clips damaged clips & the portions of clips that are not visible?