As built the maximum injector valve flow dispersion should be within two percent. Over time flow rate dispersion can digress due to deposit buildup or chatter valve wear.

The first thing to try is a bottle of reputable injector cleaner in the fuel.

If this doesn't help the injectors can be removed for cleaning, flow and leak testing if a shop in your area has the equipment.

The last solution is to buy and install a new set of injectors.

For those with lambda systems that require periodic emission testing, the above is probably why many M103 engines have high HC readings which tend to get worse as miles accumulate.

Duke