My mates '98 Escort Si, 1.6 Zetec has developed a rather annoying problem, can anyone offer some advice please?

When he starts the car from cold it ticks over horrible. When the temp comes up to normal, everything is o.k. But if he tries to drive the car before it's warmed up it's almost impossible to drive.

Last weekend we gave it an oil change, 5w 30, plus filters and changed the plugs but this has made no difference.

One last thing, the problem started just before all this white cold stuff fell on us, so don't think it's that?

Cheers in advance.