  We discuss properties of type III bursts which change sign of their drift
rate from negative to positive and vice versa. Moreover such bursts may change
sign of their drift rates more than once. These specific type III bursts were
observed simultaneously by radio telescopes UTR-2, URAN-2 and NDA in frequency
band 8-41 MHz. The negative drift rates of these bursts are close to those of
usual decameter type III bursts and variate from -0.84 MHz/s to -5.56 MHz/s.
The positive drift rates of specific type III bursts vary in the wider range
from 0.44MHz/s to 12 MHz/s. Unlike inverted U-bursts these type III bursts
still drift from the high frequencies to the low frequencies in spite of the
change of the drift rates signs. Our basic explanation of the positive drift
rate of these type III burst differs from the common assumption that positive
drift rates of Type III bursts are connected with electron beam propagation
towards the Sun. We propose that, even if electron beams move outward from the
Sun, they can generate type III bursts with positive drift rates if in some
regions of the solar corona the group velocities of type III radio emissions
are lower than the velocities of the electron beams.
