Performance issues with 75% full QNAP TS109 - but only when Windows PC is on

My QNAP 512Mb TS109 disk is 75% full (yes I'm ordering a newer 2TB model so hopefully my problems will go away), but it runs OK most of the time. However, when my Windows PC is switched on (the QNAP folders are mapped as Windows drives), the NAS starts intermittently dropping off the network, iPeng half the time can't see any players, and sometimes streamed music gets interrupted. Using PuTTY to log in and running top shows that smbd is using most of the CPU (used to be the access logger but I turned off access logging) The admin Resource Monitor shows 100% CPU for extended periods.

Sometimes it stabilises and works again (Resource monitor shows CPU dropping back to 20% or less)

So apparently, there's something on my Windows PC which is accessing the NAS a lot? I've disabled StreamtoMe, Coolio or anything similar, and I can't think of what else might be hammering the NAS in the background. My working files are elsewhere, I just have music and photos/videos on this NAS.

What tools can I use to find out which process or service on my PC is causing smbd to hog the NAS CPU?

Alternatively, there are older posts about LMS misbehaving with large numbers or files or disks getting full - are all those problems now solved? (I'm on 7.7.2 r33893)

Sorry if this is wrong bit of forum - should I have posted in Linux?