My experience with this exact type of setup has been that the seatpost has a particularly slippery anodized surface and that this rather stiff lug/clamp requires more than the usual amount of QR compression force to effect a normal amount of gripping force.

I recommend first applying something like WD40 to break down any old, slippery grease residue layer, and to also lubricate the QR lever cam with some oil.

Adding some fine grit from my bench grinder table to a thin layer of fresh grease would be the last step I would expect to take, with problem hopefully gone forever.

If the post head is actually slipping in the post I would definitely toss it. I've seen too many such slip-fit-glued or press-fit posts from this era develop cracks at the top of the post, which could suddenly cause a dangerous situation.