Hesitation

1) Props to Vince: O2 sensor is a distinct possibility. The response time of a "tired" sensor can get slow, confusing the DME about where to set mixture. Or it could be giving the DME a false "lean" signal causing DME to give too rich a mixture- not really noticeable when warm, but way too rich when cold. (btw- running too rich for too long can ruin the catalytic converter, not a cheap part) The 02 sensor is a painless replacement, fairly inexpensive part, that should be replaced periodically anyway (60-70K miles or so). Is there a lot of black soot inside the exhaust tip, or black smoke when cold? That's a sure sign of running too rich.

2) Another possibility is the coolant temp sensor; if it's defective, the DME may be fooled into thinking the engine is hot when the engine is cold, resulting in a mixture that's too lean when rich is called for. Which may cause hesitation and stalling at cold temps only. Any pinging when cold? It's located on top, towards the front of the cylinder head, near #1 intake, buried under a mess of harnesses and vacuum lines... Kind of a pain to reach. Be sure to top off coolant and bleed air out of cooling system after replacing.

3) Here's a thought, if those don't do it- a vaccuum leak at one of the intake manifold gaskets could close up when the engine is warm and parts expand. Hence- acts like a vacuum leak only when engine is cold. (Seen this before) Might consider measuring vacuum when running cold, see if it increases as engine warms... Shadetree trick: carefully spray some brake cleaner at base of each intake runner while engine is running cold, taking care not to let any get sucked into intake. If you get an rpm increase, you've found a leak. Caution, avoid hitting hot exhaust or plug wires, especially if they're old, the vapor may ignite. Check for plug wire "leaks" anyway by spraying soapy water on plug wires (preferably at night or in a dark garage) and look for blue sparks. If you see any arcing, wires should be replaced and don't do the brake cleaner test without first replacing the wires.

Let us know how it turns out.