— First of all, thanks a lot for trying to help me out. I really appreciate it.
I assume that the problem is that the player and the sine don't move 1 pixel at a time so if the player fall down for example 5 pixels and the sine go up 5 pixels, the player sometimes end up lower than the sine. Sorry, I can't explain better than that hehe :p The problem is only when the sine is going up, not down.
I created an object and pinned it to the sine and change the sine from jump-thru to solid. When the player overlaps the object I created, the solid is enabled and when the player doesn't overlap the object the solid is disabled. Take a look at the video below. The object will of course be invisible.
I wish I could find a better solution than this though but it works for now.