I have been occasionally having difficulty shifting my 1952 Chevy 1/2 ton 4-speed pickup into 4th (high) gear. It can take several attempts before it goes into gear. It seems to happen after I've driven for a lengthy distance (20 miles or so. Once in gear all seems fine. I'm able to shift into all other gears fine. I checked the fluid and it is right up to he fill hole. Any advice would be appreciated.