It could be you need a power-of-2-plus-1  for your ftable, 
when you use the interpolating oscili

See ftable in the manual. Sorry I haven't got the exact url. 

> >> <CsoundSynthesizer>
> >>
> >> <CsInstruments>
> >> sr=96000
> >> ksmps=1
> >> nchnls=2
> >>
> >>
> >>         instr 1 ;Sine
> >> kamp = 1000
> >> kfreq = 261.6
> >>
> >> aenv oscili 1, .5/p3, 1
> >> ares oscili kamp, kfreq, 1
                   ^^^
> >>
> >> outs ares*aenv, ares*aenv
> >>         endin
> >>
> >>
> >> </CsInstruments>
> >>
> >> <CsScore>
> >>
> >> f 1 0 1048576 10 1
                      ^^^

> >>
> >>
> >>
> >>
> >> i1 0.28125 2
> >> i1 2.28125 2
> >> i1 4.28125 2
> >> i1 5.28125 2
> >> e
> >>
> >> </CsScore>
> >>
> >> </CsoundSynthesizer>