peugeot 306 hdi cuts out help

iv just got a 306 2ltr hdi 2000 reg when driving a long it will just cut
out then if i turn keys it will start ok and carry on till next time has
any1 got any ideas. and also on the left side of engin theres a black box
with fuses and a big black enclosed looks like a fuse or relay that makes
a ticking noise every 10 sec could that have out to do with it any help
please.

 

Deja vu.

 

This is a VERY long shot...... Im sure some1 will have a straight answer for
you though!! My girlf had a problem in B&Q car park a few months ago, car
had been running fine but suddenly just wouldnt start, when i went out to
look it was ticking about every 10secs. Turned out to be an immobiliser
fault, she does however drive an E reg nova and didnt even know it had an
immobiliser. Worth considering anyway, perhaps the immobiliser is kicking in
when youre driving and cutting the engine off??

Andrew