Rydberg atoms of principal quantum number n in a superposition of a harmonic and a slowly varying field pass through several resonances with the harmonic field of frequency Ω as the splitting ω of the shell by the slow field varies. These transient resonances which are met when ω NΩ, where N is an integer, have been studied for the n = 25 shell of Li. Coherent elliptic states were prepared and used as initial states, and the dynamics was probed by the probability P_a for the atom to remain in the initial state. The harmonic field was circularly polarized and had constant amplitude, and the slow field varied such that ω at first decreased, then went through a minimum and finally increased to bring the atoms into resonance at two different times. This led to interference patterns in P_a(ω₀), where ω₀ is the minimum splitting. These were quite regular for coherent elliptic states of low eccentricity e and for strong fields , but less regular for large e and weak . A few states of Li(n = 25) are not hydrogenic due to quantum defects from the (1s)² core. Without quantum defects the dynamics can be reduced to that of two spin- particles and this reproduces the regular patterns quite well. A full quantal treatment, which is required if quantum defects are important, shows that the more irregular patterns are the result of quite complex dynamics involving non-hydrogenic quasi-eigenstates.