Headphoned is application that you can install from extras testing. It pauses mediaplayer automatically if headphone plug is taken off while listening music.
Does this random pausing happen while phone is in pocket? Does it happen with all songs? (like are you using mp3, ogg, flac or any other format?) Have you installed anything that might be incorporated with mediaplayer (like codecs, or any experimental devel&testing stuff). Does songs pause while listening with headphones and also when listening trough speakers.
I have that application installed. I always keep my phone in my hand when i'm listening to music. It doesn't happen with all songs. It happens sometimes with random song. It doesn't happen very rare that i can't live with it, but it's starting to become annoying. All my songs are in .mp3 format. I have Extra Decoders Support installed, but that's all that has something to do with Media Player, except headphoned. I don't know if it would happen while listening through speakers cuz i always use my headphones. And i use those i got with my N900.
Just to mention, when i had PR1.1 firmware, songs were never pausing by themselves. I think it started happening when i flashed to PR1.2.
I first reproduced it with the FM transmitter too, but it's not necessary.
If I just play music with my headphones connected it stops every 2-3 songs or so. I'm trying now without headphones, to see if I still can reproduce it.