I ran into this same problem a year or so ago. Unfortunatly I forget some of the details. Our problem was exactly as you have described. The reset ocassionally didn't startup properly, even when using a supervisor.

If you don't let the system voltage drop below 0.20 volts before powering the device back up then you the part will ocassionally not boot up properly.

Philips initially didn't want to admit the problem existed to us (cost about 2 months of OUR time), but they finally fessed up. Strangly, they have yet to document this "feature"...guess they still want engineers to fail to use their products properly.

What we had to do was use a pinch-like circuit to drop the micro's voltage down to near zero and use a reset supervisor IC. I'm not sure what the proper term is for a pinch circuit, but basically, a circuit that provides a IMMEDIATE path to ground for Vpp. You want to bypass capacitor discharge time when the power is shut off.