Sudden increase in background may have come from something you analyzed recently. I would clean everything before QID one by one to eliminate any contamination issues. Also flush some strong washout solution through the system to clean the rest of the unaccessible parts.
Rule out any vacuum related issues. If the vacuum system has a leak somewhere, or is not functioning properly, then your instrument is going to be unstable yielding high variation of sweeps, thus resulting in high RSDs. Since the background 220 measurement is based on several sweeps, the greater the variation between the cps for the sweeps, the higher your 220 background number will be. Check sudden changes in base and running vacuum pressures. Make sure those cones are good. Try new ones even if they physically look good.
It is possible that QID, cell entrance/exit lenses, or even quadrupole rods may have become dirty over time. In that case you will need to take them apart and clean them. Believe me they do get dirty even in a right angle instrument configuration.
ALS Limited, Geochemistry
Perth, WA, Australia