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@Mr_Bump
I presume you've shifted to the inner ring then undone the gear cable completely and lined up the derailleur using the L stop screw so that in 39/25 the chain barely misses the inner cage plate and then re-attached the gear cable?
This should then set you up so that the shift to the big ring works without shipping the chain.
If it still does then your chainset might well be sitting too far out along the axle OR the bottom bracket is too long?
One thing I couldn't understand though is why would you want to change from 53/12 to 39/12 anyway? It seems a very strange gear ratio change that wouldn't normally be used except by newbie's? (no disrespect intended)
Cross chaining isn't ideal anyway and puts sideways forces on your chain that you don't want both in terms of wear & efficiency.
What shifters are you using out of interest?