The first thing I'd do would be a good cleaning of the grouphead(s) and screen(s), removing the screen(s) to clean them, followed by backflushing of the grouphead(s) using backflushing detergent (A cleaner made specifically for cleaning espresso machine groupheads.). Follow the instructions on the backflushing detergent and do it multiple times until the exhaust water comes out clean. Backflushing with detergent needs to be done regularly. Weekly in a home setting is usually sufficient. Daily backflushing with-out detergent is also a good idea. Some of us do it after every espresso/set of espressos.
Java "Cimba!" phile