180-day node set by MechJeb keeps rising without any intervention

[Mokmo]

I used the Mechjeb Transfer calculator (thank goodness it works in the dev version) and set a node in 180 days. 
But something is making the node gain the necessary dv increasingly fast, and the ship is totally inert. It gained 2200 m/s while writing the previous sentence...  

I am using #585 dev version, #584 did this too. Does anyone know how i could determine what is causing this ? I doubt it's Mechjeb correcting itself because it usually doesn't touch the node once it's done... maybesome obscure game mechanic ?

Edit: Somehow if i let it do an orbit without timewarp it comes back down... 

Edited May 12, 2016 by Mokmo