Minor problem with my AD Vent Noir
 

Sometimes while pedaling, on the drive side pedal on the downstroke, I can feel a bit of play; not sure how else to describe it. Took it to the lbs a few weeks ago, and the mechanic tightened the pedal where it screws into the crank arm, checked the bottom bracket (it was ok), and tightened the crank. I've tried it with different sets of pedals and that's not it. Thanks in advance for any suggestions.

I would look at the freewheel, do you have a spare. Easiest way to check is to swap it out.

Down stroke of one side or both sides of the crank?

Were the cranks removed to check the bottom bracket?

If it is the freewheel, in general it may not coincide with the "downstroke".
Minor chance it is the chain, but that would be less predictable.

I had a "knock" in one bike's bottom bracket, yanked the cranks and took it apart, in this case it was a bad bearing. (Edco sealed bearing BB)
One other time it was debris in the bearings on the drive side, time for a repack with a very careful cleaning of the reverse rifling on the Campagnolo cups.

Check your cleats on your shoes! Occam's Razor, the simplest path to resolve a problem!

I can't help. But am intrigued by your small avatar pic. Who is that? Not Arnoux and Villeneuve by chance, is it?

You're bang on, it is arnoux and villeneuve '79 French gp.

It is only on the right side. Crank arms not removed to check bb. I don't use clipless pedals.

Remove the right hand crank arm and check if the square taper is good both on the bb spindle and the right hand crank arm. Since you have already tried to remedy the problem by using another pair of pedals, and that is not the problem, I have a feeling that it is either this fitting problem or perhaps a bad right hand side bb cup bearing. I really doubt that the freewheel or chain has anything to do with this problem. To me, it is a crank arm or bb spindle problem.

I had a weird problem on an old Centurion that felt exactly like a loose BB, but the BB was perfectly tight. It was solved by taking out the fixed cup and reinstalling it. Oh yeah, there was also a broken bearing in the BB before I reseated the fixed cup--imagine my surprise when replacing the bearing didn't solve the problem!

Vent Noir?

Please post some pix- those are lovely bikes!