Mysterious slowdowns

I am experiencing some mysterious slowdowns, especially visible in longer runs. An example when it happened (several times) is when running Soksoft's Heaven & Hell music disk on Boosted MSX2 EN.

After a certain amount of (real) time, say 50 minutes, openMSX suddenly slows down to 43 fps (87% speed) and sounds starts to stutter heavily. And after another while (can be 10 minutes or 2 hours) it goes back to normal. There are no triggers known to me that cause these transitions.

I have no idea what is causing this at all.

The lower speed is not caused by high CPU load (looks normal) or frame rate limitations of the video output. When I press F9 (full throttle), it all works as normal (goes to higher than 100% speed). When I set the speed setting higher than 100% to compensate for the speed loss, that is fine for openMSX. (IIRC setting it to 112 makes openMSX go at speed 100, according to the info panel.)

I tried with 'perf stat top' to see if there's a difference in the slow situation. There is none.

I tried to log when there are buffer underruns. In the slow situation there are definitely a lot more buffer underruns.

Here's what I tried that got things back to normal:

• muting sound
• setting sounddriver to null
• changing 'samples' from 512 to 513 (or any other higher value)
  when any of this is undone, the problem just comes back.

What I tried and has no influence at all:

• changing renderers

My last suspicion is that it is somehow caused by the audio system of my host PC (probably pulseaudio).