Obviously I suspected a hidden timing problem here as well, so just for the fun of it I used the old (unchanged) 89C51RC+ software HEX-file to program the RC2HBA and changed the X-tal for a half-value type. Guess what: still not working! So there's more problems to overcome.!
The only thing affected by the 6/12 clock is instruction execution. E. g. timers run osc/6 in 6 clock, osc/12 in 12 clock.
Philips really have to make things better! Well, they tried to, but, as we all know, backwards compatibilty is a beast.

I am currently (no 232 on board) using parallel programming, maybe Andy Ayre has something on FlashMagic in this respect.

To H or not to H that is the question.

Erik

PS I have had NO problems using the Rx2H and the 668H for almost a year.