problem with Screen('Flip') missing the next flip

1
Dear all,

I want to present a visual stimuli on screen immediately after
palying a sound. I understand the onset of visual stimuli are always
synchronized with the screen 'Flip' time, i.e. the screen refresh
time by using the command Screen(wPtr,'Flip'). So, it is reasonable
if the timing error between the onset of a sound and the onset of a
visual stimulus is within one flip interval (i.e. 1/frame_rate). But,
the fact is this timing error is always greater than one flip
interval to my experience, even if the command of Screen('Flip') is
immediately after the command PsychPortAudio('Start') in the program.
Sometimes it is even two or three flip intervals. I often got the
info 'PTB's Screen('Flip') command seems to have missed the requested
stimulus presentation deadline'. I wonder why PTB Screen always
misses the immediate next flip. Does it because the limit of the
hardware or the operating system? I run PTB on Matlab7.0 R14 on
Windows system on a IBM laptop T43.

Anybody has any ideas about this? Is there any way to solve this
problem?

Thanks

Meng