tick over

[p sev]

hi everbody hoping somebody might be able to shead some light on my problem .

the problem is sometimes on tickover the revs keep going up to about 1500rpm for a few seconds  then droping back to about 800 rpm for a few seconds & keeps doing it for a while ,

 this happens even after I have driven it for twenty minutes or more any

suggestions anyone

[oldcodger]

The obvious first  - is the air con on ?  that typically cycles the revs like that, though usually a for more like a minute than seconds.

Also, what is the engine, petrol or diesel ?  whats its service history, anything done to it recently, problem just started suddenly or slowly over weeks ?