v70 awd starting problem hot and cold

my 1998 v70 awd 2.5t has developed a starting problem! when cold esp, it is difficult to start (but always does start following excessive turning over). when it starts it gives off a sulphur/fuel smell and following 10 seconds of rough running it runs perfectly! sometimes when the car is left for maybe a week without use, it will start first time. when the car is hot, the problem is still there and still the same but will always again eventually start. the lambda sensor has now been changed twice in the past 6 months as according to mechanics it was faulty but again the warning light is on so surely this is being caused by a secondary fault...

any ideas anyone? i'm thinking maybe fuel pressure regulator or maybe a sticking pump check valve or possibly just a faulty pump (although the car does seem to be running too well for this once it has been started)....?