I had the same problems with the cable throttle on my 4* 60 ARF. I had an unrelated crash that tore out the firewall (good ole ARF glue) so I figured it was the perfect time to do away with that terrible little setup. I actually put my pushrod tube in the same place that the cable tube was, by just enlarging the holes. Besides that, my setup is pretty much the same as yours- solid wire rod, bent to fit just right. 100% reliable so far. As far as you cutting your cable tube off at the firewall, that probably wasnt the problem. I left mine long, within about an inch of the servo on one end and an inch of the carb on the other end, and it STILL didnt work right.
It's just not a reliable design in my opinion, the cable isnt strong enough.