[toni-a]

All lenses after some use will need servicing no exceptions, some might need it more frequently though.
if you have some patience, and dexterity and tools you can try yourself, while video filming the whole process to be able to go back in case anything happens.
you will also need a digital body with video output to a screen and a test chart.
mount the lens on the digital body and check for imperfections in the projected image and check the behavior, very often the problem will become obvious after that you can try repairing it yourself:
progressively de-assemble the lens (while filming the process) , check for any loose elements or free pieces of elements or screws or dirt inside the lens, clean dirt, put back free or loose screws where they are and retest, if everything fine done, otherwise it will become more complicated: most lenses have chims to adjust elements positions, in Canon lenses they are in the rear just under the mount and electronics circular plate, on other lenses each element has its own chims check for them and try to repair otherwise you will need professional help which in your case will cost as much as the lens...