the reset signal stays 5 V, goes to 0 V and the over 7 V and then back to
5 V.
When the reset is being done via the DTR/RTS signal from the serial FTDI chip, by way to the Arduino IDE or any PC application that is opening or closing that comm port, it is capacitance coupled to the reset pin by a .1mfd cap, so the overshoot to +7vdc is just a transent pulse caused by the discharging of the cap and should not be a concern nor have any effect on the chip. The reset pin unlike all the other pins is rated for a much higher voltage, +12vdc I think, because that is the mechanism to allow the chip to utilize a different high voltage programming mode. All the ugly details are in the AVR datasheet for the chip.
The biggest is that WHY Arduino
hangs when Reset or Interrupt is done during %-operation???
This has nothing to do with the hardware reset circuitry, just some software mistake or error not yet solved by you or us. 
Lefty