I tested the genes, and they work. Both they have a short impulse speed for one cycle then stop.
Could you post the bot you're working on.
If your problem is slight random movement every cycle, brownion movement is causing that. You could deactivate that in the physics tab.
Also if your goal is just to stop the bot moving the next cycle you can use toggle zero momentum with the (advanced)physics options.
Gene testing code.
cond
*.velup 0 !=
*.veldn 0 !=
start
*.velup - .up store
*.veldx - .dx store
stop
cond
*.robage 10 =
start
*.maxvel .up store
stop
and
cond
*.fixed 0 !=
start
0 .fixpos store
stop
cond
*.velup 0 !=
*.veldn 0 !=
start
.fixpos inc
stop
cond
*.robage 10 =
start
*.maxvel .up store
stop