Can anybody help with FORSCAN OBD results on a misfiring ST220?

Hi,

My emissions test are showing a high CO at fast idle, however everything else is fine. I've loaded up the obd reader and forscan and can see that my car misfires when held at around 2500 rpm (and seemingly only around this rpm range). I can't feel any misfires driving normally, and any time I've done runs with forscan on any misfires are happening around this rpm but not under WOT or even half throttle, only when held constantly around this rpm. I'm going to be doing rocker gaskets soon so will use that time to also do all inlet gaskets and retorque the cam caps, do sparks and check coil/leads to help diagnose. 

Anyway, in my forscan graphs I can see 38 misfires in just a couple of minutes. Looking at the O2 sensors, the downstream sensors are matching the upstream pretty closely - would this be caused by the misfire (I'm assuming fuel is just passing straight through after not combusting? Maybe burning up on the cat and reading an o2 level the same as the upstream?) or could this also point to faulty sensors or faulty cats? The cats were new 3 years ago, but if this misfire has been going for a while then maybe it's done these new cats in. 

Any input from people more knowledgable is appreciated

Thanks, Tom