this is it...
are you invoking an application/game that has it's own resolition settings? If you are then some monitors can change it's frequency to best accomodate the resolution, but i doubt it is this.
Based on the fact that you mentioned that the reset button does not function, i am inclined to think that there is an issue on the motherboard itself, seeing that the motherboard had power - this means that your graphics adapter has power and this will engage the monitor to come on (receiving signal), but there is no video output to the monitor...thus this would give you an "OUT OF RANGE" message, (no frequency is out of the monitors range).
If i am correct, this happens after the pc is on for a while and (usually never the same amount of time), if the pc is being used intensively, then the failure happens more sooner than later. components like you CD rom will open and close because the device is not motherboard dependant for this function to occur. Check your keyboard and note whether or not the CAPS light goes on and off when you press the CAPS button. If it stays in the same state then it means that the motherboard is not responding to this function.
I believe that you have a heating problem and your motherboard is failing or seizing all operations when you hit that critical point or there is an issue with your motherboard itself. Probably a damaged component. If i were you, i would open the chasis cover and check the state of all component - check that fans are spinning and that all components are seated firmly also check the amount of dirt on motherboard as well. Also put a conventional appliance fan to face onto the motherboard and check how long it takes for the pc to freeze again. If the pc works fine with the fan blowing onto it, then it was a heating problem, check the heat sink, processor fan and thermal grease on main processor.
Addtional chasis fans can be incorprated to assist with pc cooling, this needs some planning so that you assist air flow and not compund the problem, arrange the fan(s) so that you achieve maximum heat dissapation.
Good luck