QUOTE

If your CPU or disk usage climbs too high you are likely to experience audio 'buffer underruns' as clicks or pops in the live sound from FL Studio. The good news is that CPU and disk usage can be lowered considerably if you take the time to make some adjustments to the FL Studio Audio settings. The Audio Buffer is a temporary store of audio waiting to be sent to the soundcard, allowing FL Studio to even out momentary spikes in CPU load. If the Audio Buffer runs dry, because your CPU or hard-drive can't keep up with the 'real-time' audio stream, then your Soundcard will make rude pop, click or stutter noises. It is worth noting that underruns can ONLY occur in real-time playback, they will not happen while exporting to wave or mp3 file. If you do hear that sort of thing in an exported audio file, then it is likely a plugin behaving badly, check the plugin settings in that case.

QUOTE (yordan)

Have a look here : http://flstudio.image-line.com/help/html/app_underrun.htm

They have a nice underrun optimization topic, depending from your audio drivers.
Maybe this can help.
By the way, did you change something in your hardware, namely your sound card or your sound adapter?
Regards
Yordan
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