Audio wobbly when transposing

This is new to this programme and started suddenly about 2 months ago. Every time I transpose, even one semitone, the audio goes wobbly like its underwater. If I reverse the transposition (change pitch without changing tempo), the sound is perfect again. Any suggestions as to what is causing this? All other sound programs on my OSX 10.7.5 are working fine. Thank you.

Effect > Change Pitch and the others are speed-happy. They were designed to produce OK results rapidly. Not so Effect > Sliding Time Scale… You can sometimes produce much better results slowly with that tool.