I know that this is a dead thread, but I only recently downloaded the P-61C and encountered the "bouncing" problem discussed here a couple of years ago. As I don't see any reference to the fix that I found, I am posting my experience here for future visitors.
I installed the P-61C in P3Dv4.5. When parked, it would skitter and slide around the ramp. It was even worse when I had it parked using Where Are My Aircraft and viewed it from another aircraft. It would launch itself hundreds of feet into the sky and gyrate wildly. Quite annoying. It happened with no other airplane.
Having heard that this kind of thing can be caused by a contact point error, I opened the aircraft.cfg and checked it. One value in the contact points jumped out at me as being obviously wrong. The damping value (10th parameter) for the nose wheel is set to 12.980. The actual allowable range of this parameter is supposed to be from 0 to 1! Since damping relates to how much the airplane bounces when landing, I reasoned that such an outsize value might be bouncing the plane violently into the air, or at least causing it to jump around.
So I reset the value from 12.980 to 0.980, which is consistent with the values for the main landing gear. Specifically, go to the first "point" line under [contact_points], which in the stock aircraft.cfg is:
point.0 = 1, -7.261, 0.000, -7.400, 2000, 0, 2.75, 45.000, 0.148, 1.863, 12.980, 6.000, 2.500, 0, 175.0, 190.0
and change that 12.980 to 0.980.
So far this seems to have completely cured the problem and the P-61 sits nicely on the ramp like any other plane.
It's hard for me to believe this wasn't spotted during the work that was done to cure this issue a couple of years ago, but I don't see a reference to it in this thread, and the latest version of the P-61C available in the Library appears to still have the incorrect value in the contact point.
Anyway, I hope this helps other late-to-the-party downloaders enjoy this excellent airplane.
August