It is probably just the cold weather causing the gaskets/seals to contract or the metal portions to contract. This sort of things happen every year around this time. One other thing you can change, if you haven't done it in a few years is the IAC valve but that is a pain for the Zetec.
Edit.
Check/clean/change the IAC valve.
IAC cleaning
from Ray's UIM/LIM how to in Duratec Maintenance how to.
"Replacing the IAC gasket:
(From IAC faq)
Over time, the IAC (Idle Air Control) valve becomes sticky due to buildup from the EGR and PCV systems. It can stick partially open such that idle quality is adversely affected, and engine RPM's will not drop below 1500. For a while, blipping the throttle will restore normal idle, but this only works for so long.
The fix is to remove the IAC valve by unbolting the two 8mm bolts that hold it on and disconnected the wiring plug. If you are careful enough, replacement of the gasket will not be necessary. The gasket costs less than US $1.00, so it is no tragedy if you destroy it. Once the valve is off, spray it out with some carb/intake cleaner and make sure any gunky residue is gone. Reattach, and you are done. It is a lot cheaper than a new IAC, which runs around US $70. Note: Ford claims the IAC cannot be cleaned, but this is not true in my experience." Ray.
Location of the IAC for the Zetec.
"It's on the back side of the intake plenum under the throttle body area." DESIGN.
I looked at the Haynes manual and the Zetec IAC valve is located next to the starter. Below are the steps from the 1996 Ford CD to remove it .
Idle Air Control (IAC) Valve
Removal
1. Disconnect battery ground cable
2. Remove the air cleaner outlet tube and resonator assembly.
3. Disconnect the main chassis vacuum line.
4. Disconnect the ignition coil and radio interference capacitor wiring.
5. Remove the fuel line bracket retaining bolt.
6. On 2.0L engine equipped vehicles, raise vehicle on hoist.
7.
7. Disconnect the engine control sensor wiring or fuel charging wiring (depending upon application) from the idle air control valve (IAC valve) at the idle air control valve connector.
8. On 2.5L engine equipped vehicles, remove retainer for engine control sensor wiring from the upper retaining stud bolt.
9. Remove the two idle air control valve retaining bolts or stud bolt (depending upon application).
10. Remove the idle air control valve and idle air control valve gasket.
Installation
1. Note: If scraping is necessary, be careful not to damage idle air control valve or intake manifold idle air control valve gasket surfaces or drop material into intake manifold.
Clean idle air control valve gasket mating surfaces.
2. To install idle air control valve, reverse Removal procedure. Tighten the fuel line bracket bolt to 8-12 N-m (71-106 lb-in).
3. On 2.5L engine equipped vehicles, tighten the retaining bolt and stud bolt to 8-12
4. On 2.0L engine equipped vehicles, tighten the idle air control valve retaining bolts to 8-11 N-m (71-97 lb-in).
