E21 m50 idle issues

K I have a m50b25 I'm my car and it has started to do my nut in.

It starts no problem at all but it idles about 1500 rpm to 2000 rpm when cold for about 2 min and just shuts off then. Then I restart it and it idles between 1000 rpm to 1500 and shuts off. If I keep the revs up a tad it will stay running. I have tried the air flow sensor and that is perfect. If I disconnect the tps it ticks over perfectly so is this the problem. Im not convinced it is. I do know that I discarded the charcoal caster and that plug is doing nothing should I have it plugged in.