Master Limiter Causing Distortion - help?

[Lin]

I'd be really greatful if someone could help me with this

I'm trying to master this track and get it up to commercial levels. However, I've noticed that whenever I turn my master limiter (JS: LOSER/masterLimiter) on and bring the threshold down to anywhere that causes any reduction, I get a clipping/ditortion noise when playing in reaper and after rendering, most noticeable on the vocals and guitar solos (2:10). However, I can't see any clipping on any of my tracks or the master. I'm getting it on my headphones too, so it can't be my monitors.

http://soundcloud.com/linrichardson/...-sky-tech-demo

I've not had this problem on my previous projects. The only changes I've made to my process are to add some makeup gain on the tracks and to switch over to my new USB audio interface. And i don't have this problem if I go back to old projects. It's pretty heavy on the effects (60), but that hasn't been a problem previously either. The guitars especially are compressed to hell, partly in a failed attempt to address this.

I just can't isolate the source of the problem. If someone can get to the bottom of this I promise to offer my first born child to them as a blood sacrifice.

Running Reaper 64, Windows 7, Native Instruments Komplete Audio 6, Guitars are line in from a POD, vocals on samsung usb condenser mic, drums sampled, synth are VSTs. The limiter on the linked track applies max reductions of around -3db.

Other more general production comments also welcome.