Transmission Stumble

I just recently added a 2000 crown vic PI to my collection. Car has 125k and is in reasonably good shape. However the transmission seems to stumble and hesitate when shifting. Its most noticeable when your cruising along and lightly press the gas. when it attempts to downshift it stumbles before eventually dropping into gear.. if you add more gas and speed up quickly it will shift into gear without the stumble. it holds all of its gears while driving. any suggestions?i have also just replaced both the VSS and OSS to deal with a speedometer issue i had which actually ended up being a loss of power to the motor in the speedo ..

I can be wrong here as you car has a bunch of computers including ones for transmission shifting. But to me sounds like it may be a vacuum leak as it shifts properly when giving additional gas, Hope it helps