Ihave a 200 20hp merc when under full power some times it jumps out of gear but when I let off it picks back up again what could this be.

A misadjusted or binding shift rod can cause unit not to fully shift into gear,has impeller been changed lately?

No I have not changed the prop yet but I have a smaller one that I can try.

He was talking about your water pump impeller,because the shift linkage has to be dis connected when it change,thinking someone might not have adjusted it right.
Prop that you are talking about, could have spun the hub,When it first goes bad it will slip at high RPMS but if u let off it will grab,look in the hole in the middle and see if it looks like the rubbers broke or burnt looking.when they first go sometimes its hard to tell.