audiounit playback callback (render callback)period uneven sometime ?

I use audiounit （subtype:vpio),to play the audio .

The data is sent to my playback program via netEQ.

The audio sent over the network is relatively uniform,
But the video is jittery.

question:
1 .The audio unit starting time is not fixed，sometimes 250MS，sometimes 400MS (in different iPhone iPad )
2 . the playback callback period not even sometime .such us 8K Sample_rate, Most of times are called (playback callback ) in more than 20 milliseconds.340~342BYTE. but
occasionally, the callback time interval 130+ ms,And ioData->mBuffers[0].mDataByteSize= 340 byte alse. I didn't make the time-consuming operation in the render callback function . Just copy the data to iodata. and data is enough.

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PlayCallback(void *inRefCon,

                               AudioUnitRenderActionFlags *ioActionFlags,

                               const AudioTimeStamp *inTimeStamp,

                               UInt32 inBusNumber,

                               UInt32 inNumberFrames,

                               AudioBufferList *ioData){copy data from my buffer }

• ------------------------------------------------------------I want to know what will affect your render callback cycle(period).