Dodgy Idlding

[vince n]

Don't know if it is a problem. My 98 450 is fine, all of a sudden I started it for the first time in a week and it reved to about 1800 rpm and would not drop. After about 15 miles I stopped the revs still up on tick over, I then turned of and re started it went back up to 1800 then seemed to sort its self out and went back to 900 rpm on tick over, anybody any ideas what that was about I would be gratefull, Le Mans looms and nothing must spoil that.
Many thanks Vince Narduzzo