Swede, This is a copy of a Tuning manual I use on the Acroloop equipment. It has a built in 'scope in the software so you can see imediatly what the effect of tuning is.
Although the parameters used here may not be exactly the same as is used in your case, it does give a pictorial view of what the effect is on changing tuning parameters and curing instability etc.
They shoot for a following error of less than +/- 5 encoder counts for the entire range of RPM.
The feedback encoder must yield at least 4000 counts/motor rev. Also the torque mode of operation is recommended.
The PDF file is too large so here is the link
ftp://www.compumotor.com/bbs/acr/torque.PDF
Also see a very usefull ac servo Handbook ( you have to register with the site) by Sanyo Denki
www.motiononline.com
Al