1997 Festiva Trio S high idle

[bionic2006xr6t]

Hi guys hoping someone can help me out.

I own a 1997 festiva Trio S with the B5 engine which has developed a high idle when cold. We aren't talking a few 100rpm but 2500rpm to 3000rpm. She is long in the tooth being in the family and having traveled 460,000km. The way I have been combating this ATM is I let it run for a few minutes or drive it down the road and then turn it right off wait 15 to 20 seconds and restart. It will then return to normal 500-600rpm idle. Ambient temperature dose not affect it weather it's 30c to -2c. If Istart a could engine it will happen 8 out of 10 times. Warm previously run engine it never happens. I have had it back at ford and we swapped out the computer with no affect. They fiddled with a few things and then told me its old buy a new one and that I have had a good run! Anyone out there got any idea's? I would like to get it sorted.

Cheers Ryan