Clogged injectors aren't as common as they were in the '80s, the EPA requires injector cleaner of good concentration to be in all gasoline sold in the US. I haven't seen a clogged injector in years. Non-ignition related skips and missed are usually due to bad injector seals or wiring. Air leakage around the injector is also a common problem. On the B234, burnt/sticking valves usually cause such problems. Volvo no longer reccomends the use of injector cleaners, because of reformulated fuels. On the B6304 960 engine the usual problem is either a bad injector seal or bad manifold gasket. The only way to clean those injectors if they were to get gunked up would be to remove all of them and send them out.