What the gauging system and operator cannot control directly are the high-speed STMD variations and the profile streaks.
How can you isolate and define the sources of STMD variations and profile streaks?
Observing the STMD cycle(s) can often pinpoint a source of the variation--the periodicity of the cycle can usually be correlated with a process parameter.
If the process is more complicated (such as a coextrusion line with multiple extruders), the STMD variation may consist of thickness variations from multiple sources, each having its own period and amplitude, all comingled into a complex in-line display readout.
Once you've determined the predominant source(s) of STMD variation, you need to put on your process engineer's hat to determine its actual cause.