my EF 100mm f2 has problem focusing on low contrast objects and eyes

[theorium]

I have an EF100mm f2 that I use on my EOS 6D (also taste on EOS500D). there are a lot of instances when it goes on an endless hunt for focus and never stops till I move it. it starts going in and out. it mostly happens on low contrast objects but it happens on high-contrast things too, which the most annoying one is focusing on eyes.
I want to know if it's a common issue with this oldy lens or if I should send it for repair.

[theorium]

one shot , 9 points (using the center) no filters,
based on the questions everyone is asking, I am assuming the lens has a problem.

[p4pictures]

There is an EF 100mm f/2 USM that was launched in 1991 - it's similar to the EF 85mm f/1.8 USM launched in 1992. They are both different to the macro 100mm lenses which were all f/2.8 maximum aperture including the most recent EF 100mm f/2.8L Macro IS USM.

[deebatman316]

Yes I am aware of that that. The manual for this lens is no longer available anymore. It's design is based off the EF 85mm F/1.8 USM lens. It is also not a Macro lens either like its F/2.8 counterparts. My uncle used owned the original EF 100mm F/2.8 Macro lens from 1990 it used an Arc Form Drive (AFD) AF motor. Not the newer Micro Motor seen in newer designed lenses released in 1993 onward.

[jrhoffman75]

Maybe this:

01_CT1-8536_ENG.pdf