Did you try the "mice in a haystack" thing nums sugested ?
Just place an obstacle across the screen splitting it in two, and move it aside now and then to mix the bots, or if one side got whiped out.
Also you should consider what kind of bots you're using, it may just be a virus from one of the other bots.
Also using .vshoot inc will send out a very weak shot, so the most likely mutation I can thing of would be breaking the condition, so .vshoot inc is called while the virus is loading, so if you're channeling a big gene by the time you're ready to fire vshoot will be rather big. Not sure if this is how it works, but last I tryed evolvig the wiki bot it evolved into moving fast forward while reproducing so the first offspring would get in the way and alow the line .repro inc to accumulate a big enough sum to make a sizeable offspring.
I guess my point in that would be that 10 (Or more) .vshoot store would be more stable.
The last virus alge I posted worked a lot better, but deleting random genes when infected doesn't seem to be the best solution for evolution to work. Gonna try spreading viruses a lot slower this time, and maybe use slime as the genome is getting too long.