I created a mission where the outside temperature was minus 6 degrees, thick cloud cover base at 8,000 feet and a snowstorm.
When approaching the top of the cloud cover from a high AOA manoeuvre or split S descent the plane started to oscillate violently along the longitudinal axis, is this an attempt to model icing or something else?
Unlikely. DCS does not model icing (yet!) What was your speed at the time? The viper tends to do that when stalling out.
I was doing north of 450 kias! I think it is related to very low temperatures, I am going to try and recreate the scenario but increase the temperature to above freezing!
Did you have your refueling door open?
No, clean loadout, no exterior fuel tanks, air brakes not deployed. I have never encountered the problem in over 700 hours of F16 flying, I am sure it is related to atmospherics 
DCS does not simulate such atmospherics yet. Really.
Yeah, no offense but it seems like a PBKAC.
EDIT: I’m ready to eat my foot in case I’m wrong. 
EDIT2: Also Longitudinal axis? Wings rocking left and right uncontrollably? Which frequency?
DCS simulates air density changes based on temperature, has done so for a looong time. Not saying that that caused the issue, but it’s not impossible. Compare takeoff performance of any plane in 30°C vs -30°C, you’ll see a huge difference.
No. It oscillates along its LENGTH, like see-sawing
PBKAC - No!
And what has Eagle Dynamics said about this? Where can I find the warning in the TOs regarding a clean >450KCAS Jet???
Ok, one thing at a time- then the oscillation is along the Lateral Axis ok.
With what frequency does this oscillation happens and with what amplitude?
You managed to recreate any of that?
EDIT: usually the axis reference is like this, that’s why I asked.
Any chance pitot heating was off, and the airspeed indication was wrong?
It’s quite possible for there to be some weird issue if that was the case, I doubt many people fly in adverse weather often enough that if there was a bug in that code that it would be found right away.
Offering a non-weather related problem path.
The pitching is along the longitudinal axis, pitching up and down over the centre of gravity with sudden dramatic losses of airspeed, you can observe the violent pitching motion in a F3 flyby.
I’ll try to recreate it tomorrow and make a vid
You pitch in the lateral axis. Longitudinal is when you bank/roll
Hey I’m not trying to pick on you but, look at the image I’ve attached.
When your nose pitches up and down you are actually rotating along your lateral Axis.
It might seem counterintuitive but it’s like that.
If you create a mission where that occurs, attach it to a post here se we can download it and test too.
Also try to report all valuable info on how you enter that oscillating behavior.
The horizontal stabilisers are moving up and down. Is the stick moving when this is happening with no input from you? I only ask because its FBW so if the stick is moving and the horizontal stabiliser it suggests a physical hardware input from outside the sim rather than the sim itself.
If the stick is static, it suggests a problem within the sim.
Upload the mission file from that, and a track file too. We can see if it is reproducible.
Yeah, as @Victork2 said if the tailplanes move there’s something driving them.
In the Axis control page scroll through the whole horizontal line and remove any possible input.
There’s two controller fighting for the pitch input.
It’s not a Sim problem.
Edit: also, yes it’s oscillation along the lateral axis.