Column shift C4, original linkage behind a 302. Engine and transmission have about 10k miles on them. This has happened twice in a month. Once about 700 miles into a 1200 mile road trip to the Roundup, and yesterday at a car show. On the road trip we stopped for gas and drinks. Got into the car, started it up and put it into D, hit the gas. Nothing. Revved up like it was in N. Went up and down the gear shift, nothing. Put it back into P, popped the hood and confirmed the lever was moving on the side of the trans. Started it back up and it went into gear. I thought I had a loose shift rod nut, so I tightened that down and we finished the trip without issue. Yesterday I went to a car show and parked for about 4 hours. When I hopped into the car to leave, the same thing happened. I put it back into Park and revved the engine up, then dropped back down to Drive and I was in gear ago. Drove home without issue. Kickdown functions as it should as well. When the issue is happening, no gear matters from R to 1. The fluid is full and I did a fluid and filter change (I had a leaking pan gasket) last year with nothing to report besides some sediment in the pan. I also put a bottle of correct LubeGuard in it. I do not believe this transmission has a shift kit, but it was a fresh rebuild in 2015 and was filled with 20W Dexron III/Mercon at that time - I used the same when I did my fluid change.