firmware can choose the resolution,

I have seen this quote many times on the web that Marlin can (The advantage is the firmware can choose the resolution,

and can perform the arc with nearly constant velocity, resulting in a nice finish. Can some one tell me where this setting is?

I have been getting good prints then bad ones. Seems that the printer does not hold consistant velocity while printing it will do some of the printing very nice and smooth then
out of the blue it changes velocity and then back to normal print speed. It will do this al all sorts of places on the print. I have looked through the code and there is no change
of feed rate so I am assuming there is a setting in the firmware of Marlin that needs to be adjusted. It seems to do the change on speed mostly on arcs.

any help here

Ron

It's your slicer that determines the speeds at which you print. The firmware limits those speeds to whatever you have set in the Configuration.h if it tries to go past them. Slic3r and Cura have settings whereby if a layer will take less than a certain amount of time, they will slow the printing speed to allow for the filament to have time to cool. That might be why you see some different speeds, although the slicers generally have different speed settings for inside perimeters, outside perimeters, infill, solid infill, top solid infill, bridges, etc. Some also usually print the first layer slower than the rest.

If you are having problems with rounded portions of your print, you might want to look at the xy jerk settings. And possibly just slow the printing speed if necessary.