could be the crank sensor. if it misses a beat it could cause backfire (metal tube sound?) and engine stalling.

did you get any sort of check engine light?

1000rpms at startup and 700rpms when the coolant reaches a certain temp is indeed withing proper idle speed parameters for an M50

other information would help, as much as possible.

what kind of motor oil, how many miles since it was changed.
how long since last coolant change.
auto or manual transmission.
recent maintenence.
etc, etc.

if you have not changed your spark plugs in a while, pull them and check for fouling. large amounts of carbon deposits could be responsible for causing preignition, which would cause the knock sensor to dial back the timing and significantly reduce engine power output.