The more likely answer is no, it's not your grinder. Or more accurately, it more likely that your grinder is not the sole cause. Typically sour = brew temperature too low or extraction is too fast, bitter = brew temperature too high or extraction too slow. So what to make of sour/bitter? Well, it's hard to say what's going on with a pressurized portafilter, but it's possible that the coffee is both under and overextracted, with a falling temperature to boot.
I know, I know... not very helpful.
You might start by paying attention to the boiler cycles. Small espresso machines often have large temperature deadband (*), leading to unpredictable brew temperatures unless you engage in "temperature surfing" (i.e., timing the start time of the extraction dependent on the heating cycle) or other trickery like flicking the steam switch to force the brew temperature up. These machinations are sometimes necessary to get the correct brew temperature, and unfortunately they're highly machine-dependent, so I cannot give you cookbook instructions. If you're lucky, someone else has figured it out and published their findings; if you're unlucky, you get to map it out yourself with a stopwatch, a fast thermometer, a Styrofoam cup, and a lot of time to kill.
Getting back to the basics, you'd be well served by reading Jim's The Home Barista's Guide to Espresso
, especially the section Diagnosis of Extraction Problems
. It covers the gamut of problems, but keep in mind that his diagnoses assume non-pressurized portafilters.
(*) Brew temperature deadband = the difference between the thermostat's on and off temperature.