EOS 10D Error 99 outdoors

Background: I have owned EOS cameras for as long as they have been manufactured and have collected a few lenses over the years.
I bought the EOS 10D three weeks ago.
My workhorse lens is a pre ultrasonic Canon 28-70 zoom.
This works fine indoors with built in flash of ettl flash (I have had to buy as my old one was attl!)
Problem: Using the same lens outdoors I get error 99 with annoying frequency.
Firstly I thought it was the heat.
Now I think it is the bright light.

Any ideas?

I can't help you mate

I've also just had my first Err99 problems. I was shooting outside with a 10D/Canon 28-135 IS, fill-in flash (built in) and I got the error three times. Each time, I had to switch the camera off and back on.

It's not happened once since but it does make me feel a little bit wary