I am a new convert to Pure Music with version 1.8. I was also experiencing the sames problems and was able to solve the problem by disabling the DSP options.
However, I also found enabling the "Dim" option in the Monitor (as described on page 8 of the manual) with the DSP options still active had the same effect.
Before I found this solution I took all the steps advised in the manual under the "Optimizing System Performance for Audio" section.
Hope this helps.