Independent of why the lever gets hard to swing at a point is that these levers are old enough to have had their factory lube dry up. I suggest doing the "flush and lube" service on them to help prevent the common pawl gets sticky on pivot issue these levers are so well known for.

If you do this servicing and the fresh lube gets to the lever's internals you might also find some of the friction you feel in the lever at that certain point is lessened too.

I find it far easier to remove the lever/shift pod unit from the lever bodies (which Shimano calls "brackets" IIRC) and these Flight Decked type will have a ribbon wire running from the lever pod to the body (and thus to buttons and head unit). While the ribbon can be retained few use Flight Deck heads any more, do you? If not than just cut the ribbon close to the shift pod and body. I also like to use compressed air during the flushing and relubing. Andy.