The hissing noise could possibly be some king of air leak, you normally get some king of air rushing sound around by the throttle body as the air is pulled though the small paths in the idle circuit. IT could all so be the the fuel/air ratio (mixture) is set up wrong,
you say you been restoring you car, have you changed any engine parts, like fitting an uprated breather system..... as this can have quite effect on the base idle...

It sounds as that is starts from cold OK but as it worm's up and comes off cold start the engine then cuts out, i would say the most possible cause is air leaking in somewhere

let me know
Cheers
Steve