It's actually what divides so many solid alliance opportunities. Russia wants Poland or the player is otherwise bored and goes to war with Prussia over the whole thing -- hoping the Austrian will look the other way while Prussia is brutalized for a few months.
In 1805 I typically suggest a Prusso/Austro/Russo alliance through at least the first, if not the second great war with France.
Basically, pound France twice hard and then everyone is on more equal footing.
It's another reason I like 1792. There is no mandatory bad guy. There are numerous possible major player (at least on the land) and Britain has even less of a stranglehold on the sea.
This leads to a more free-form style of diplomacy and not so repetitive.