problem with starting and stopping

Hi everybody have a renaulte espace 2.2 dt diesel 1998 model i have a problem with starting the espace it takes me 3 times before it starts this was fixed before 5 months ago by changing the heaters and it went well for some time.. this time though after driving for sometime it turns off regularly turning the key to the off position.. after some minutes though starting would be ok but putting it off by turning the key wouldn't stop the engine it continues running even taking off the key and pressing the lock on or off. it stops with a some tries. anybody had experience of this before please help... thanks