The millisecond component of the Jovian decameter emission has been studied at high resolution, in order to examine the short-term behavior of the S-burst drift rates and to define the drift rate spectrum at high frequencies. By using dynamic spectra having 300-&mgr;s 3.3-KHz resolution and covering discrete frequency bands in the range 26--33 MHz, large systematic changes in the magnitude of the S-butst drift rates are observed to take place on a time scale of seconds to minutes. It is noted that the drift rate variability is of sufficient magnitude to prevent reliable evaluation of the drift rate dependence on frequency, unless simultaneous or near-simultaneous multifrequency measurements made at frequencies within 0.6 to 4.4 MHz of the electron gyrofrequency at the Jovian foot of the Io flux tube. This result is interpreted in terms of an ionospheric electron source, rather than within the conventional theoretical framework of trapped particles initially accelerated at Io. Low-resolution intensity-time tracings reveal a characteristic organization of S bursts into groups, each of several seconds duration and consisting of tens to hundred of bursts. Abrupt changes in mean drift rates from group to group suggest that some of the group structuring may be a source-related rather than a scintillation-related phenomenon. Finally, some evidence exists for systematic variations in the group-to-group mean drift rates, which may be due to predictiable temporal changes in the magnetic field gradient on the Io flux tube. These temporal changes are due to motion of the Io-associated magnetic flux tube in the Jovian magnetosphere.