Sometimes the door will not open (interior door handle) on occasion. This has started recently and I wanted to fix it before it gets too cold out (no garage to work in).

When the door doesn't open the handle is at the halfway point and it will not push back any further.

I have both Bentley and Haynes manuals. I have looked over the diagrams but I am figuring when I finally open everything up I won't be able to find the problem, since the problem only occurs occasionally.

Any tips on which part to focus on from anyone that has had this occur would help.