crankshaft sensor

[318isturbo]

hi guys been a while hope all's well
here's the deal once my car get's warm when i stop in traffic or at the lights after a few seconds when rev's are at idle level it suddenly goes into as i can only explain (LIMP) mode where it start's to die the rev's drop and it feel's like when a cam belt or battery dies no power and nothing you can do it will die some times it will start staight away other times takes a while and i get a loud back fire from the exhaust.(FUEL)i guess.

what does a crank sensor do when its on its way out has anyone
ever had to get there's replaced
ive got no idle control valve no cam sensor where im running m/s management and changed the tps.

A-TECH(ant)
has told me to change the crankshaft sensor which i belive will work but wanted to find out some tell tale signs see if it sounds right
CHEERS PEEP'S

[318isturbo]

or possible dme relay but has only been changed about 5 months ago no longer than that and was a main dealer one two anyone ever changed a dme relay and had it go so fast