I decided to raise the questions here, to see if others encounter the same behaviour:
 at startup: MP comes with a splash screen, where you see the progress while it loads alll the plug-ins / modules. That splash screen is surrounded by the standard window frame, saying "MediaPortal Splash Screen - Not Responding". But all works fine, it loads OK, and it does NOT take longer than usual. I would say that 1 in 20, it does NOT show the error. It always goes past this, it does not crashes on start-up. I would disable the splash screen if it is possible...
 when changing channels: sometimes, it temporarily switches to a completely white screen (with no sound) with the standard blue window border, with the title "Media Portal - Not Responding" and the spinning Vista cursor in the center of the screen. It never crashed, it changes the channel after all, but it takes up to 4-5 sec. I would say it happens mostly when switching TO tv3 or FROM tv3, but it did it on other channels as well. It happens maybe 1-2 times every day but it appears to be completely random, as it did not do it for the last two days... When this happens and it switches to the next channel (after those 4-5 sec), the image is frozen for another 2 sec, but the sound starts straight away, and then the image starts ok, in perfect sync with the sound. It is like the image would "wait" for the sound. I always use CH+/- on the remote, I do not change channels pressing the correspondent number (it works, if I want, but it is not very convenient).
 watching recorded shows: go to "recorded TV", select a show from the list, press OK and nothing happens. You can navigate through the list or leave that menu, or play ok other files, but the selected show does not play. If I go to watch tv and back to "Recorded TV", the same show would play flawlessly straight away. This happens occasionally, maybe 2-3 times a week. I record a lot and watch the shows next day. This enables me to watch two series in 90 minutes, not in 2 hours. (30% of the watching time is advertising !!!). I do a defrag of the hdd used for storage every two weeks (it is like 40% full, only).
Any ideas if I can do anything to improve any of the issues, especially no.  ?