Electromagnetically induced transparency (EIT) resonances using the 5S1/2 → 5P3/2 → 5D5/2 ladder-system in optically thick Rb atomic vapour are studied. We observe a strong line narrowing effect and gain at the 5S1/2 → 5P3/2 transition wavelength due to an energy-pooling assisted frequency conversion with characteristics similar to 4-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 85Rb 5D5/2 hyperfine structure even in the presence of strong power broadening.