Jerk and stall when put into gear?

Make sure the idle is correct first. If it's too high, it will do that to a certain extent. Your's sound pretty severe to me. You might want to get the rear wheels off the ground and with the brake off, see if it will go through all the gears and if it will come back down to idle when you apply the brakes. If it jerks and bucks like a manual trans car, it might have a bad converter. The no power and backfiring indicates to me that there is a tune-up problem. What were the compression readings? Did you do a valve adjustment?