Hi. My name is*****'m a camera repair technician with over 20 years in the business and I'll be glad to help you with your question today.
It sounds like the "flex", (the flexible ribbon cable), that connects the power diaphram has a crack in it. This is a common problem with Canon lenses. It is repairable and the part is available from many sources but the lens needs to be completele disassembled to repair. This really isn’t a user serviceable problem and should be repaired by a qualified technician. Below is a link to a directory of repair shops, one of which should be in your area and able to help you.
Thanks for your comment about the flexible ribbon cable on my Canon Lens.What you say very much agrees with the problem I have experienced and you say it is common occurrence in Canon lenses. I shall be glad to get it repaired as I am fond of this versatile "L" lens.I am naturally pleased with your answer.Thanks &best regards Keith
Thanks for this advice. It could save me a lot of trouble. I still see 24-105 lenses on sale so think it is still current. I am a member of a good camera club and I am sure some of th top guys there will advise on who can arrange the repair. I shall however get on to Canon to make double sure it is still current ( I must say a replacement L lens like mine might be pretty expensive). Thanks for everything and for first class advice. I thinkthat's it best rgsrds Keith