...What I would propose is that inflation should rise as stability rises (controversial, I know). As inflation rises, it becomes more and more likely for a downturn to ensue (which would be a steady drop of stability, and the bigger your peak, the lower the trough). Only stab. level -1 or -2 should have no inflation, with inflation becoming negative at stab -3. This would encourage a fluctuating market, as occured. Btw, no inflation is a bad thing- people will spend less, and so the economy tends to hit a brick wall, and deflation over a long period is even worse.
Therefore, you have two options- you could go for high stab when you really need it, but this would require long term investments- it should be hard to change, but there ought to be a reasonable automatic investment. The other option is to aim for a steady, stab 0-1 economy most of the time. You'll get less money this way, but lower inflation.
To semi-balance this, every time production or trade tech goes up, you could get an inflation reduction- rather small, but enough to make a difference. This would have the added advantage of keeping prod and trade tech useful later on.
To add to this, it should be hard for EVERY power to budget, particularly for war which is far too cheap. Maybe something like administration costs to keep the budget lower, I don't know. But an idea might be that some time after going to war, your stab will improve to reflect a war economy (more likely to occur at lower stab.s). But after war exhaustion reaches a certain point, or after the war, your stab drops quite quickly.
Maybe all this is not so fun, gameplay wise, but I think it makes far more sense than the current system. Sorry for the long rant