Cutting out at optimal temp.
 

Hi guys I just got myself a 2003 and I run in to small issue basically car starts first time when cold no problems and runs spot on till it gets to operating temperature once its there engine cuts out completly new engine was fitted 4500 miles ago
any ideas what it might be?

first welcome to the club
second if you haven't yet read the new owners thread about 10 times then re-read it New/Potential owners start here
third there is no such thing as a "new" engine just rebuilt/reman
fourth are you sure that the replaced engine has sufficient comp#s and was properly installed?
fifth how are you watching the temp? is it the dummy gauge on the dashboard?
sixth will it start again when it is warmed up?

Must be on limp mode, for how long and how many miles have you driven since this happened, any CEL? Have it read.

just speculating here. To me it sounds like it might be dying when it switches from open to closed loop. Which happens as the car is warmed up. Closed loop is primarily driven off of the O2 sensors.