Either NorTech do solid pins (which are also a bit longer to fill the whole slot in the wheel) or Associated sell upgrade solid Factory Team pins, part number 1654
I just use the standard split rollpins in all my cars and don't have any issues. On the older Associated cars before the cone washer was introduced if you clamped down on the wheel too much it would bend the pin and weaken it. Crank down on a wheel more then you normally would then take it off and check to see if it bent the pin, if it did you need to space the driveshaft closer to the centre of the car. Move some of the shims from the wheel side of the hub to the CVA side, that should sort things out
What wheels are you using, they might have a shallower slot for the pin than normal that's putting excess stress on it?