blunty 07

My bro had some welding done to his mk2 fiesta and ever since its being running rough. its got a xr3i efi engine in it. the battery was disconnected when the welding got done. when you drive it form cold it runs fine but when it gets hot the idle speed drops to 500/600 rpm and starts chuggering and wants to stall. iv changed the iscv,map sensor,throttle posistion switch,fuel pressure reg,air temp sensor,ecu and still no change. If you take the pipe of the top of the fuel reg it then runs normal. any ideas ?