What's the condition of the transmission fluid? With 280K miles on the clock a more appropriate question might be how dirty is it? How does it smell compared to fresh fluid?

Check the fluid level. Go for a 20 mile drive, pull over on level ground, I run the lever through the gears stopping a few seconds in each position but I don't know that this is necessary, pull the dipstick and wipe the last few inches with a clean lint free cloth, insert the dipstick and pull again to check the fluid level. It should be between the min and max marks. Adjust as necessary. It's only a cup or two between min and max.

For kicks, bypass the transmission vacuum amplifier; the blue flying saucer (BFS) by the glow relay. Find the black vacuum line from the transmission amplifier to the transmission; it'll have a green cylinder just after the amplifier. Find also the output of the vacuum control valve (VCV) on the injection pump. Guess what? It'll also have a green cylinder just after the VCV. Connect the transmission line directly to the line out of the VCV. Keep only one green cylinder and don't change the direction of flow through the cylinder. See if that changes shifting behavior.

Slipping under any condition isn't good. Slipping with fluid at the proper level is very not good. Try what I suggest and report back.

Sixto
87 300D