Recently, my RS seems to start fine ... then after about 3 mins or so the rev's die down untill the engine cuts out.

It'll do this until the temp needle is past the black block at the bottom of the temp guage.

Any ideas ?

 

Check your fueling and timing m8.

 

Sorry to sound a dumb blonde ... but how would I go about doing that ?

I just find it wierd how it suddenly starts doing this.

 

Take it to a garage and ask them to do it.