a few comments
1) 'testing' ISRs by setting the flag is not worth much, no randomness will occur. A typical example would be the 'passig a short phenomena' which will never show up if the bit is set by C.
2) my way of generating artificial interrupts is a clipwire on the interrupt pin moved acoss a grounded file.
3) This might be an artifact of the ICE I am using Did you set the breakoint inside the ISR or did you step, some ICEs skip ISRs when stepping.

Erik