The modulator uses engine vacuum to tell the valve body when to initiate a shift, and if the vacuum is low, like when you open the throttle, and there is a leak, as soon as the shift begins the load changes on the motor, changing the vacuum at the modulator, and the tranny becomes confused, not wanting to upshift until you close the throttle and a strong vacuum returns. So you get that where you have to feather the throttle to get it to shift, and it wants to jump outta gear when you accelerate.