One thing that hasn't been mentioned is card thickness. SD cards should conform to certain very tight tolerances in terms of physical dimensions but they do vary in thickness slightly from brand to brand. If a card is too thin, then there is insufficient spring in the contacts to give a reliable electrical connection. I haven't seen this problem myself but have read about it elsewhere. I have never had any problems with 2Gb SANDISK SD cards, which I usually use. Also, it helps to clean the contacts by polishing them with a tissue. If all else fails, I'm afraid it means a trip to the service center.

I don't work for Line6 so I have no idea what it costs for a repair. You will have to speak to the service center for an estimate.

Modern amps are modular, just like computers. The Spider Jam is a series of seven circuit boards interconnected by ribbon cables. The most common fault is with the ribbon cable connectors which can shake loose or just become tarnished by their environment and connections become less conductive. This can happen whether you use the amp or not and a humid sub-tropical climate is more likey to affect the connections than a dry one.