Error 99 on a Canon 10D

[rdjones]

I have been on the North Humberside Stages today and on 3 different occasion I have got the dreaded error 99, when it happen for the 3rd time I thought I would head home. The weather today was dry and very dusty and only seemed to happen when I had my Sigma 28-135mm on.

When I got home I have given the whole camara a good clean and I have rattled off about 500 test shots, using both my lens and using different memory cards with out any problems.

I have done a quick web search and I haven't anything that will help me further. So has anyone got any idea is it just a one off or is it something more serious?

I am going to Donington tomorrow so I am going to keep an eye on it and I might take my old Fuji S602 with me for backup.