Extreme stiffness during "ons" can be due to an excess of levadopa in the brain, that activates all muscles at the same time. You may have to try smaller, more frequent dosages.

As to Stalevo, I take Stalevo 50 and can't say I like it too much, most of the time I have to supplement it with 1/2 a tablet of Sinemet 25/100, because it stops working when I eat something (regardless of the food's protein contents). I believe I'm clearly undermedicated, but... my neuro has told me not to take more than 4 x Stalevo 50 a day, in other words, not to exceed 800 g of entacapone, for whatever reason.

As to balance: there is orthostatic hypotension that makes us lose our balance when we get up from our seat or out of the car, and then there are the balance/vertigo problems derived from bad lighting conditions (we walk much better in the sunshine than at dusk) and from our inability to move our eyes fast enough to take in and adjust our balance to our "mobile" surroundings, i.e. walking on crowded sidewalks or inside shopping malls, on a beach, etc. can be really difficult.

In my opinion, daily balance exercises (yoga, calesthenics, Tai Chi...) make a great difference.

Take care
Teresa