Your hinges are worn out. When that happens the first tell tale sign is the hood sits up in the back after you close it. Don't bother trying to make any more adjustments as they won't hold or as you've seen, things get worse.
The other thing to be careful with is if the hinge is real bad, the rivet can break or pop out. You don't want that to happen!
If the hood is hard to lift, especially with well lubricated hinges, the only thing that can be is the springs are weak. Not much rocket science involved there.
Glenn's post makes perfect sense. 67 and 68 hinges are the same. 67 and 68 hinge springs are very different. 67 springs are bigger because the hood is heavier. Compare springs between the two years and you'll instantly see the difference.
Weaker or worn springs in 67 hinges might not work out all that well for a 67 but might work fine in a 68. 67 hinges with strong 67 springs might not work out all that well in a 68 as the hood will want to lift up too fast.
A good tight set of 67 hinges with either worn 67 springs or good 68 springs would make your day.
I have some used 67 hinges. I might have a set of NOS hinges but not completely sure. If you decide you want to try another set let me know.