mine would have problems with that until i converted to a 66 style throttle linkage where it is mounted somewhat on the intake manifold, it works a lot better and it is pretty solid! check that stuph.
Jack up the car, crawl underneath, see if there is a cable or rod between the transmission and the throttle linkage. It should be connected to a small lever on the driver side of the transmission, next to the shift linkage. It may have come loose or it needs to be adjusted. It is not difficult to adjust...lengthen or shorten the linkage and then road test to check for the kick down.
I guess I should ask...did your car downshift before?
Readjust your linkage,
Easy set. Prop throttle full open,
with cable unhooked (I do the front) press trans lever forward till you feel it engage, there should be spring travel slightly afterwards, hook cable back to connection leaving half the spring travel.