Have a 250SE 1968 (w108 i think??) that starts fine when cold. After 15/20 mins car dies. Will start again only when cold (and first time). Have been told that it maybe the cold start valve/device. Is it possible to test this? Any help much appreciated.

CSV is only a starting aid device that squirts raw gas into the intake.
If the car is stalling as it warms up, I would first do a fuel volume test.
This can be dine at the line to the CSV.. disconnect the line, slip a rubber hose over it and put the other end into a measure container, and turn the Key ON to energize the electric fuel pump. ..you are looking for 1 litre/15 sec flow rate with NO air bubbles ..a clear hose is nice for air evidence, or submerge the hose exit end to accomplish the same..........
Seee what you have...

You have described the same symptoms as what happened to me quite a while ago in my 250SE. The cause was a blocked fuel filter.
The test Arthur describes will most probably reveal a lack of fuel flow after a short period of time.

I would replace the filter, I had the same thing happen with my 220SE, or it might be the inlet screen at the fuel pump. The screen can be cleaned with spray carb cleaner.

A few things come to mind. Possibly a coil though slim chance. If the car is of US origin, you may have two solenoids at back of pump. One is fuel shutoff on decel. The system fails and shuts off fuel to the pump. The valves may not have been adjusted for many miles. As the aluminum head grows, valve clearance decreases.

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