Some trouble with a simple measure

[allthumbspiano]

I'm working on Grieg Sailor's Song and have everything fine with the piece except for one measure.  I don't have the sheet music in front of me right now but will update with the number.  The measure is the one with the thirds played in the right hand and I just can't get the thirds down.  I can do it fine when not played as a piece (when I am just practicing the thirds, even with the left hand) but when I try to play it with the piece the thirds just fall apart.  I've been giving it some time and it hasn't improved, I even got rid of the slur and am playing them staccato as I find thirds to be easier that way.  I know that this is a technique problem and am wondering if anyone knows a way to fix this.