Stuttering and buffering on streams is normally due to insufficient bandwidth. Please see the Recommended Bandwidth FAQ page here (quality is based on the connection rate). If the bandwidth is fine, exit the game and re-launch it. If that does not help, exit and then re-launch the MLB app.
If bandwidth is a problem, power cycle the router and modem to try to improve the connection. (Power Cycling is turning off the power to the modem and router then powering back on, in an attempt to refresh the Internet connection.) Also try to limit the amount of devices sharing your Internet connection as this could have an impact on your overall bandwidth.
While on WiFi you can improve your signal strength by moving your router closer to your device. Raise your router. the height of your router will usually improve its radio signal performance. Be mindful of anything that could interefere with the signal like cordless phones, baby monitors, etc. Also try limiting the amount of devices sharing your network.
Latest Version of the MLB app from Google Play or iTunes
Latest firmware update of the Chromecast device. You can check that here.
Latest Version of iOS or Google Play services installed on mobile device
If you are having issues accessing your stream, please verify your user name and password on MLB.com. Then log out and log back into your device in the "Settings" menu of the MLB app.
Try a hard reboot of your Chromecast device. Remove your device from its power source for 30 seconds, and then turn it back on.
You can also reboot your device from the Google Home app. Tap the icon of the Chromecast device, then go to Settings > More > Reboot. Tap Reboot again to confirm.
Try plugging in an HDMI extended cable to move your Chromecast device away from the TV and/or speakers.
Move other WiFi-enabled devices away from your Chromecast.