The spikes in data could be Outliers. Check if tgey are outliers using a Boxplot and, if they are, do a study on those specic data time, operator etc. If you conclude, those outliers are not possible, then you can remove them from the data set and then calculate the Sigma ( standard deviation). You will have a reduced standard deviation tgat reflects fairly the process SD.
Another method is to sub-group the data with a sub-group size of 3 or more to get pooled standard deviation which is likely to represent the process’ SD more correctly.