Why is it you are getting pulled?
The Police Officers that pull you must give you some reason. It is unlawful for them to simply pull you for no reason at all. It is usually because you have committed a traffic or other offence or they suspect you may have committed something similar.
Stopping someone purely to check their driving documents are up together is not usually a valid reason on its own unless they have information about a person or a vehicle to justify a stop. If this is not the case then they may not be acting lawfully.
When you get stopped again you are entitled to know why you have been stopped or their grounds for stopping you. e.g. poor driving, car not up together, too noisy etc.. If you feel that you have been unjustly stopped then you may have a case, particularly if it is a frequent occurrence.
If you are serious about the complaint, I would start recording each incident with as much detail as possible, take down collar numbers and names of officers and witnesses, time and date each incident and particularly record the reasons why you have been stopped and any actions that were taken. If you have been committing offences each time then you havn't really got a case. Once you have all of this you will be well armed to speak to your duty Inspector at your local nick and if there is no joy there, seek further advice from a solicitor.
The bottom line is, the Police have to justify everthing they do - if they cannot justify what they have done then they are in the wrong and you will have a good case against them. Good luck.