What do you mean "reset automatically"? Presumably you're referring to Live Chess, and the way it remembers your previous selection for the time control? Is your question therefore that you like the time control to default to a fixed time control, even if you choose to play a different time control like 5|0 for a specific game?
Timer effectivity

The timer in Live Chess does count down continuously, but sometimes the time remaining is adjusted for lag - see the first FAQ I posted.

That would be just a matter of setting up a custom time control that adds 300 seconds for each move:
As a custom time control, though, it would be unlikely that anyone would be automatically matched with his game request - someone would have to see the Open Challenge on the Seek Graph and choose to accept it, so you could be waiting a while.
Given that an average game can take 40 moves, we're talking about 200 minutes here, or almost 3.5 hours per game - I'm not sure if he'll get many takers for that length of game...
I hope you will add new rule on timer.
When i set 5 minutes, i hope it will reset automatically on next step.