Get rid of those tires b4 you end up on your head with the bike riding you!

i did notice one thing that carries from cars to bikes on asphalt you're kingpin angle is not steep enough basically that is the angle from the top arm to the bottom throught the spindle it will cause the fron end to act real funny on pavement optimum kingpin angle is 15deg hope that helps

your setup looks to be correct, if it darts when you let off the throttle than your frontend is misaligned but as you said, it happens while on the gas, it is swingarm or axle bearings you need to be checking

I had the same problem w/ the moped tires. Let some air out and fixed the problem.