Andrew,
Welcome to IIS
As Steve said try using longer lenses like 25 or 15mm with 9mm as the shortest one for general use.
As the scope is of unknown pedigree I would take it to your local astro societies viewing night and try some other eyepieces to see if that is the problem or if the mirror is not up to the job. At least that way you can test it for no real outlay. Then, if its the eyepieces follow Steve's advice remembering that 150x will be your max magnification on most nights so don't get carried away with short length lenses.
Cheers