Slow Tranny Kickdown

Hi,was wondering why sometimes my tranny will downshift all the way to second sometimes under full throttle while passing,almost instantaneous, and other times(mostly),it takes a couple seconds or more,up to 5 to do it.Sometimes it won't at all.I am starting to get the hang of the shifter,but it would be nice not to have to move it at all. Acts like it maybe an electonic glitch more than a vacuum one,but I suppose the modulator could be sticky. Any advice would be appreciated. [V]