Thanks for getting back. Sample rates are all set to the same parameters. Today, everything's working great. Run thru about 10tracks, and applied a couple of ozone mastering plugins to each. The only thing I've done differently, is closing down each project before opening another. In the past on my pc machine, I've simply opened a new project, and I figured the old project was closed by default. Could this account for anything? Do you know if in mac Reaper projects remain open when a new project is open? Maybe today is just an anomaly, intermittent problems are tough to track down.

Man this weird. Worked great all morning. Closed down Reaper and jumped on safari to send the last post. Closed down safari, and opened Reaper back up. All the tracks which played fine before are all glitched out now. Monitored cpu, and everything is running very low (below 10%). Been all over the Reaper forum archives and web searches, and I can't find anything to remedy

I think you might be right about the problem being outside Reaper. I monitored the tracks with the performance meter when Reaper was running fine, and now when it's not. This odd. These readings are from the same track, same plugin settings, exactly the same... Reaper running ok CPU 9% - 10% RAM 330 FX CPU 6%-7% Reaper running with glitches CPU 5% RAM 410 FX CPU 20% Seems like I'm getting a huge spike in the FX CPU usage for no reason. Almost like there's something running on the dl in my computer. This is a little off a normal Reaper question, but is there anyway I can monitor what's going on as far as what's running on this mac?

Activity Monitor will have the bottom line (and should agree with Reaper's performance meter). Those numbers are of a computer pretty much at idle though. I suspect you have something going on with your interface selection/control. The above scenario sounds like when you opened Safari, the system audio was set differently than your Reaper project and ended up creating a conflict. Then when you went back to Reaper, you were unintentionally sample rate converting everything on the fly. That you don't see any high CPU activity on the meters doesn't exactly follow that theory... Maybe something like one of the plugins you are using has a problem with live sample rate conversion. All speculation without enough info on my part but that's the kind of scenario I'd look for. These scenarios can get tricky... 10 years ago, incorrect settings would shut you down. With modern computers and OSX and apps like Reaper, the system keeps up with the first 3 mistakes you make in setup and then the 4th crosses the line. Then you aren't sure where to start.

Nice work! You diagnosed the problem. My usb audio device (Steinberg ur44) works great in Reaper, but when I open system preferences>sound it doesn't recognize the inputs or outputs of the device. I shut down the mac and let it sit for a few minutes. Turned it back on and REaper works great. From now on I'll use my pc for internet and keep this mac for music. Thanks again! I really appreciate the assistance

Cool! Just FYI Between OSX Audio MIDI Setup (your OS audio utility app), Reaper's control panel, and the ability to make aggregate devices, you can have a very convenient setup with OSX that covers everything you want to do. You just need to be aware of the control panels and make intentional settings.