Canon 100mm USM Macro Aperture Problem

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Having an issue with my macro lens, seeing as the repair shop isn't open today I thought I'd see if anyone here may have any ideas on what's going on/what it's going to need done to it.
The problem I'm having is - if I stop down to, say, f14 and take a series of shots some of them will obviously be at f14, but there are then a lot in there that randomly seem to be wide open. The camera still tells me it was taken at f14, but it's clearly not. Seems to be absolutely random, I can fire off a continuous burst without changing focus and within that there will be some at the "right" aperture and some that are wide open. AF/MF doesn't effect it neither does the specific aperture it's set at.
I've cleaned the contacts and tried it on another body, which is giving exactly the same issue. Anyone got any ideas of what the problem is?
 
could be oil on the aperture blades randomly causing them to stick. normally with aid of a torch and doing some longer exposure test shots where you can wtch the blades in action you can see if there looks like any oil build up.
if there is then its a repair centre job to have them cleaned.
also stick it on f32 and then use the DOF preview button and do this several times. you should get a very small pinhole in the aperture but if its sticking and staying open it could be an electronics malfunction too. quick google seems to bring this up as a fault a few otehrs have had.
 
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