Pencil erasers can remove and clean, but rubber is an insulator. Don't go crazy with this, but just a bit of 91% rubbing alcohol on the end of a Q-tip applied gently to contacts will remove oxidation and improve electrical contact. Do the lenses first (I assume more than one). If this still doesn't help, then the problem is with the camera. This is a service issue (repairman). If you only have one lens, try it on another camera. Since you say "sometimes" I think cleaning will do the trick.
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Make sure the auto-manual focus switch on your camera is set to auto (A) and also make sure the auto manual focus selector on the lens is in auto (A) as well. If your lens contacts are dirty then this will affect the function of the lens as well. Clean those with either a lint free type cloth and alcohol or methylated spirits.
This to me is a connection problem and may only require moving the AF-MF button back and forth a few times or removing the lens and cleaning the contacts between the lens and camera body. Generally the "L" series lenses are pretty resilient in construction of offer the user few problems. The lens contacts can be cleaned with a soft red rubber pencil eraser where you lightly polish the contact area of the gold contacts. Clean away any rubber dust with a lens cloth not a tissue, be extra careful not to get dust into the camera body cavity. reassemble the lens and try the camera lens operation Diagram
99' indicates some problem with the AF contacts. This is a common error
in Canon lenses. It could indicate a very simple problem like dirty
contacts or a much serious one like the breakage of USM cable. As a
first step, you can try cleaning the lens and body contacts (the metal
contacts between the lens and body) using a pencil eraser. Be careful to
hold the body facing down so that no dust falls inside. After cleaning
both the lens and the body contacts, re-attach the lens and try
shooting. If the AF does not work still, you need to take the lens to an
authorized service center. For the jerky focusing ring, you need to get the lens serviced by an authorized service center.
This usually happens when the contacts between the lens and the camera are dirty. Try taking the lens off and cleaning carefully the metal contacts on the camera and the lens. If that does not solve the problem, you can get a warranty repair or replacement since that camera should still be under the original 12 month warranty.
It's probably broke then. Clean the contacts to the camera and try again. If it still don't work you can probably be sure it is likely to be a bit faulty. Check the camera has auto focus mode enabled if appropriate.
Did it ever work on this camera?
I have seen some Sigma lenses that are auto focus on canon film cameras but then when put on the digital they dont work right. If you have used it on this camera then make sure the lens is clean.sometimes dirt on the front wil make it try focusing on the dirt. Next try focusing in manual focus to make sure the focus is working. Then take the lens off and then put it back on and make sure it clicks into place.
Let me know if none of those things work. also make sure to try focus on something close and somthing far away.
First make sure contacts on lens are clean ( see enclosed pic #2 ) . If this does not solve the problem the ultrasonic motor may be faulty pic #1.( sometimes switching on but sometimes not , will have to be replaced , unless there is a bad connection between the motor and lens pc board ) #1 #2