Arduino UNO Timer issue
At 9600 bps, it takes over 1 ms to transmit one character, so getting just "Prog" in 5 ms sounds about right. Try increasing the serial port speed and/or sending shorter messages (e.g., just one character per timer tick).

I don't know much about how the Arduino Serial module works internally, but my guess would be that you're simply overloading its transmit buffer with all the stuff you're printing, and it's just "giving up".