Yes - I fixed it tonight. Basically, after doing some research I tried updating the firmware in the remote and that cured the problem. You need to use a 'delay between repeats' of about 300ms though.
I also had a similar problem with controlling volume on a Pioneer TV via RS232 (irrespective of how long I pressed the vol+ control, it went up for 2 seconds). Apparently the original TSU9600 firmware didn't handle RS232 repeats correctly.
I don't understand why Phillips are still installing the old defective firmware on new Prontos, because the current version was released in November 2006...
So it's working fine now. However, I'm still not convinced that the RS232 control necessarily gives any real improvement over the RFX9600 IR blaster output. When you use the cursor to scroll through the EPG, it moves more slowly using RS232 - that might be seen as an advantage because it makes it easier to see what you're scrolling through. I guess RS232 also allows for tidier wiring.
