Please explain what kind of work was done in tunning??(like plugs, oil change service, distributor cap, etc)
RPM fluctuation could be due to irregular fuel pressure. When was the last time you changed the fuel filter. Fuel pump could also be the problem.
How is your alternator condition?. Some times at around 187,000 KM you need to check them for voltage verification, fluctuation in voltage will also cause in RPM fluctuation.
Also clean the Mass Air flow sensor. Clean it with a sensor cleaner spray.
Change the air intake filter if its not changed in regular service.
1) Check fuel pump, replace fuel filter
2) Check alternator voltage (verification).