Dang! Nice job! The battery is 9V?
I bet you will see that too much current is going thru the PNP transistor.
Do you have access to Logic Level MOSFETs? They have very low turn-on resistance, like in milli-ohm range, so will not dissipate a lot of power.
Say it had Rds of .005 ohm and you had 400mA thru the motor : P=I^2 x R = 0.8mW
The PNP: P=IV, so 400mA X ~0.5V (from C to E) = 200mW. Big difference.