it sounds like there's some distortion in the frame's seat tube at the joint. There are two possibilities and the signs will be slightly different for both.

If it's only local distortion, which is fairly common at brazed or welded joints, then the post will bind as it comes down to the distorted area, but the binding will be fairly constant as you work the post into the area.

If the tube is slightly bent at the joint, which also happens, then the post will start to bind, and the binding will get steadily worse as you push the post deeper.

The former is easiest to correct, and most bike shops have reamers that can handle this, though the may not have handles, or extensions able to reach deep enough. If the binding is fairly slight you may be able to improve it yourself by using medium grit lapping compound, and working the post to lap out the binding area. Lapping compound comes in pint cans but if you have a good relationship with an auto mechanic or engine rebuilder you might be able to "borrow" a teaspoon, though it might pay to have a beer in hand when you ask.