"RESET -conected to Vcc using a cap 10uF(as in the data sheet) "

This is one solid example where the data sheet is bluffing ! MCUs with Flash do not work reliably with Capacitor resets.

Use a reset controller like DS1232 or select to suit your convenience from a whole myraid of these chips that are available. Search this forum with keywords like "Reset Controller" for more help on this.

If you care to read more about these flash MCUs you will come to know the various internal processes that happen within the chip as soon as you power up before they decide whether to run the code they already have or wait for new code from you. And it is precisely in this time that the capacitor reset lets you down - the reset pulse it generates is not accurate enough and repeatable enough to allow the process proceed smoothly.

Use a Reset controller.

Raghu