Cold start - rough idle and hesistant throttle

Car is a 1999 20vT Plus, driven almost daily. On cold starts, I'm having a rough idle and hesistant throttle, for a minute or two. Today it stalled and started back only after 5-6 attempts.

I've seen a lot o posts, with different side effects and/or general condition of the car in question. I've booked the car to friend's garage next week, but it's the only Coupe he ever worked on (mostly newer diesel cars, 4x4s, vans), and I would appriciate a direction or list of items to troubleshoot, from simple/most likely further down...

ISSUE
On cold start, idles roughly, revs to about 1.200rpm, falls down, occasionally almost stalling, for about a minute or two. During this time, it's also really hesitant and unresponsive on throttle. It's been happening for months, it's worse when cold and really damp. Today it actually stalled for the first time, after running for 10sec, and wouldn't start back for 5-6 times.
Warm or hot starts are normal/excellent, and so is general driving. Mpg is good.

CAR / ENGINE CONDITION
Coupe 20vT Plus, 1999, 142.000km (88.000mi), mostly stock, daily driver.
In 2016 refurbished turbo (stock but fitted with 360deg bearing).
In 2017 (at 117.000km / 73.00mi) it's had a partial rebuild of the engine, due to excessive oil consumption - changed all the bearings, piston rings etc. It's been fitted with head with enlarged/polished ports (unknown spec, stock valves etc.) Stock map - no chip/remap. After initial run-in, oil consumption is non-existent, less than 0,1 lt/1.000km. Mpg improved - moderate driving, I get around 25mpg in city driving and over 30mpg on A/B roads. Other than the mentioned issues with cold starts, works and drives great.

Has this issue just started or has it been there ever since the rebuild?

After the rebuild it's been hunting revs a bit (let's say 800-1.200rpm) when cold, for a minute or two. During that time, it responded to throttle a bit lazy, but not hesistant.

From 6 months ago (last winter - cold weather), it gradually started getting worse, revs dropping like it could stall, more and more unresponsive and jerky to throttle. Maybe a misfire, but I'm not sure (can't recognize).
As I said, yesterday was the first time it stalled, and wouldn't start back. Battery is in very good shape.

Once warm, after a few minutes, it's ok - normal partial or full throttle response etc.
No "engine check" or other lights.

Hi,
I've dropped it at the friend's workshop yesterday evening.
He started it this morning, after sitting outside overnight, and it worked perfectly, no misfires or stalling In Coupe's defense, it's been really nice and dry for the last 2 days.

That sounds so typical of a Coupé

Is this the list of things to check?

1. Idle control valve - if faulty/dirty, would give rough idle regardless if cold, warm or hot?
2. Spark plugs
3. Ignition coils and cables
4. Injectors - wouldn't give problems on cold start, more on hot start and wide throttle???
5. ???


Forgot to write earlier, for ages car has had atmospheric blow-off (Forge dual piston).

1. ICV (idle control valve) - mine did this when cold. Very nearly stalling. Was OK when warm. The ICV regulates the idle when warm as well, so when moving off from lights (even when warm) it may tend to not feel smooth (hesitant, lumpy or lurching) as the idle gets too low when you balance throttle and clutch as you move off.

Try cleaning the ICV first. Mine did not have huge amounts of deposits, but it idles well now. You may well need a new gasket as the old one will be broken up or in pieces as you take the ICV off.

Update / report.
The car's been back for a week, I wanted to drive it for a few days before getting back.

The car is running perfect!
Spark plugs have been changed, to NGK Laser LPG1
ICV and throttle body have been cleaned.
Cables to coils are in good condition. Coils were not changed.
A small boost leak has been found and patched - small cut on the air hose before plastic pipe that goes to throttle body.

Cold starts are "by the book" - revs to 2k, drops to 1200rpm for a minute or two, settles to around 800rpm.
Warm/hot idle is smooth, 800rpm, dead stable.
Hesistany to throttle when cold is almost gone.
Could be placebo, but I think car is also much smoother when warm. In all situations - part throttle modulation is better, pickup to throttle variation is immediate, acceleration is stronger.
Boost leak didn't help, but I think it was mostly down to the spark plugs.
I've started this thread because I initially discarted spark plugs - I thought the car had Iridium plugs installed during engine rebuild (2y / 25.000km ago), but it was fitted with Denso standard plugs (model K20TT).

* spark plugs fitted are NGK Laser LPG1 (n.1496), because it was listed by spare parts catalogue as a replacement/upgrade over "standard" BKUR6ET.
I would have been happier if they fitted BKUR7ET (n.7873) or Iridium BKR7EIX (n.2667), as per Fccuk recommendation, but the garage only asked me on the phone if I want std. or Iridiums.
NGK Laser LPG1 were 40% more expensive than BKR7EIX.
I hope there souldn't be any issues down the line, on a 20vT engine?