I work the same way as you, programming in MasterCam, post to Xilog and then routing with SCM Tech Z30 that has Kvara controller. (Moldmaking) Do yours machine has kvara or NUM?
Like you I also has problem with the machine shaking. I usually run a filter in MasterCam so it create's G2/G3 where possible, it's getting better but not quite ok.
Lately I also have problems with the machine does not follow my G2/G3 code as it should...(Are going to start a thread about that)
In Xilog there is something called "jerk" that I think affects how the machine reads the code, if you set the value right the machine should "float" properly.
BUT I had a technician from the company that sold the machine to me in my workshop a few days and he created a program in Alphacam which we tested to run but the machine shake lot. So he began to adjust the "jerk" but after many test he give up and adjust the lenght movment from 1mm to 10mm... The result of that was the mold was more angular then round..and the machine still shake
In MasterCam can you also set "highfeed" which adjust the feed so the machine "float" better. I've never tried it myself but I know works very well on other machines.
What version of MasterCam do you use?
Maybe not so much to help butter maybe something.
Sorry for my bad english!