Car stops without warning

That model unfortunately has many issues it cost Mercedes millions and spoilt their brand, everyone i know with that car had problems. Unfortunately no one could ever pinpoint it exactly, although it believed that its because it was the first generation of computer controlled cars that was the issue, basically in easier terms.... At one point Mercedes made everything electronic in their vehicles go through one computer, now in modern day cars you've got different computers and sensors on every single electric component. My mate didnt lock his car because it A) didnt reopen but started the car

Or B) it just wouldnt open or start again. He loved the car so learnt to live around the problems. But to this day Mercedes have no idea other than the what i stated about 

If the crankshaft sensor or its wiring are faulty, it can send a signal to cut off the engine, take it back to the Mercedes garage or a good independent garage. It is unsafe to drive and could cause a accident/engine damage and cost you a lot to put right. Stay safe and healthy and good luck.

At the risk of advertising another forum (albeit in a helpful way), I'd suggest visiting here: Mercedes SLK World and asking the question. I've been a member there for nearly 3 years with my SLK and in that time I think they've saved me hundreds of pounds in garage bills with their knowledgeable advice.

An afterthought to my previous reply.

A '99 SLK would be the R170 model with the flip out key yes? I seem to recall having a similar problem with my 170 about 15 years ago. It turned out (unbelievably) that the flip mechanism within the key itself was worn and depending on which way you put the key in the ignition, it could 'flop down' and kill the engine.

It sounds unfeasible but it did actually happen. My solution was to make sure I put the key in the same way every time.