mine also did this.. change the coil pack and the king lead... it turned out my coil was earthing out and breaking down and failing.. causing loads of airbox popping action, misfiring and general engine armegeddon.. the other problem mine had was a knacked hall sender... take the dizzy cap off and look for debris... and oil deposits.... the seal goes on the back of the dizzy and it just coats the hall sender in the dizzy with oil and crap.. also check the 3 wire connection to the dizzy, this is prone to getting all mucky etc.... but the fact the car runs for 45 odd secs sounds like its the coil.. as it breaks down when it gets warm, as its earthing out... so go to it! :wink:
and if your anywhere near brighton and want a hand... or ideas etc... let me know!