I've noticed that, especially on flyer bases, that the model can get wobbly. I attribute this to the lack of stability int eh base. I've even had the base flex so much that my basing sand cracks off! To fix this, I like to glue some bits of old sprues on that bottom.
In order to do this, you will need to snip off any bits off the top and bottom of the sprue so that it does not stick out too far. After that, file both sides flat.
Lastly, glue the bits of sprue into the bottom of that base. It might be a smart idea to file everything flat so that the base lays evenly. The best way I have found to do this is to use some double sided tape and tape a piece of sandpaper to a piece of cardboard or a table that you hate. then move the base in a circular motion to even everything out. Be careful! you can end up sand the base uneven.
I find the end result a lot more stable.