What on earth is this ship doing ? Bug?

[CitizenVeen]

So I am encountering a really strange situation.

I have a rocket which starts out 1.6 TWR, ramping up to 3.00 while the SRB burn. After it drops them it has 1.7twr with 2 engines. Climbing till 5 when they burn out. Then second stage, transorbit burns etc. Its about the first stage.

Seems a perfectly smooth TWR curve. When I launch the rocket everything seems to be going fine untill it reaches 270 m/s (about ~30s into the launch). Then its speed stalls, and actually starts to drop steadely. Even if i keep pointing the rocket straight up during the entire flight.

There is no loss in thrust on any of the engines (neither the solids nor liquids). MechJeb and Engineer tell me my ship is above 2 TWR.

If the weight of the ship is dropping (the fuell is burning) but the thrust stays the same, and I am reaching a higher altitude, it seems phisycly impossible to me that the ship would actually loose speed So I guess its a bug, but I no idea whats going on. Maybe FAR is suddenly calculating and insanely high drag on the ship? Something like that?
here is a picture, as you can see the TWR is 2.3 when I took it, the surface (as well as vertical speed of the ship) is dropping at this point.

 

Anyone any idea whats going on? Or what addon is causing this?

 

[Ultimate Steve]

A bit of an odd question, but is there any wheels or landing legs on this? They tend to cause glitches like this.

[CitizenVeen]

nope, no legs or wheels

[Nansuchao]

maybe you have an engine in the fairing firing in the opposite direction after a stage event? Just guessing.

[CitizenVeen]

Nope neither.

It was a bug, I did solve it though. As you can see the rocket had a bit of a weird shape, after changing to much bigger fairings (a less optimal rocket though, because of the added mass), the bug was gone. I guess FAR got confused by it. It did land on the moon in the end