Hi, I've recently acquired a 4 string Burns Marquee bass re-issue (I'm guessing about 10 years old). As I move up the neck the E and A strings start to produce an oscillation or "throbbing" effect similar to when you tune by harmonics, except I'm only playing on one string. By the time I get to the 12th fret it sounds as though there are two strings sounding an 1/8 tone out - almost like when you turn the "detune" knob on a synth - but I'm only playing one string (and yes I'm making sure the other strings are properly muted). If I unplug and press my ear to the body I can hear the oscillation effect on the E and A kicking in about the 9th fret, but not the D and G. I changed to new strings (flats), no difference. What might be causing this? Is there a cure? EDIT:My bass has a fender-style bridge with grub screws either side of each bridge saddle. If the grub screws heights on each saddle are not even, could this be the cause? The E grubs are noticeably out from each other, but the others seem balanced.