  Electromagnetically induced transparency (EIT) resonances using the
$5\rm{S}_{1/2}\rightarrow5\rm{P}_{3/2}\rightarrow5\rm{D}_{5/2}$ ladder-system
in optically thick Rb atomic vapour are studied. We observe a strong line
narrowing effect and gain at the $5\rm{S}_{1/2}\rightarrow5\rm{P}_{3/2}$
transition wavelength due to an energy-pooling assisted frequency conversion
with characteristics similar to four-wave mixing. As a result it is possible to
observe tunable and switchable transparency resonances with amplitude close to
$100\%$ and a linewidth of 15 MHz. In addition, the large line narrowing effect
allows resolution of $^{85}$Rb $5\rm{D}_{5/2}$ hyperfine structure even in the
presence of strong power broadening.
