Arrows that are PORPISING - correction advice needed

[Yazoo Bowhunter]

My arrows are porpising (like fishtailing, but moving up and down in flight and not side to side) in midair during their flight to my target.

A)  Why?
B)  How do I fix it?

This problem just all of a sudden started as it didn't use to do this.  Any explanation to that as well?  I know it's not my arrows either, as they have been matched up perfectly to fit the bow and me...

[Tim Schoenborn]

It's your nock point........

Your nock has moved or the string has broke in.

[mnbwhtr]

Nocking point is correct?

[Dirty Bill]

Your nocking point is most likely too low. If thats not it,your arrow is hitting something on your shelf.