ok i have a mk2 1.8 auto gl - haha auto

Anyways this is my problem that im getting and if i could get some pointers as to what it may be at least i wont seem too much like a naive plonker at the garage.
Im driving along and the car begins to loose revs, maybe a min later the revs drop completly and i have to pull over because i've lost all power in terms of moving.
I'll try and start it - it trys to start but cant, so i leave it for bout 5 mins and make sure everything is turned off and i can suddenly start it again.
This has only happened so far when i have my stereo on and maybe some other stuff like lights ect, that may just be pure coincidence tho.
This is probably one of those problems that could be a number of issues but at least it may paint a better picture for me.
Many thanks.