  Transmission electron microscopy is one of the most powerful techniques to
characterize nanoscale magnetic structures. In light of the importance of fast
control schemes of magnetic states, time-resolved microscopy techniques are
highly sought after in fundamental and applied research. Here, we implement
time-resolved Lorentz imaging in combination with synchronous radio-frequency
excitation using an ultrafast transmission electron microscope. As a model
system, we examine the current-driven gyration of a vortex core in a 2
$\mathrm{\mu}$m-sized magnetic nanoisland. We record the trajectory of the
vortex core for continuous-wave excitation, achieving a localization precision
of $\pm$2nm with few-minute integration times. Furthermore, by tracking the
core position after rapidly switching off the current, we find a temporal
hardening of the free oscillation frequency and an increasing orbital decay
rate attributed to local disorder in the vortex potential.
