variable tickover

[xxxscimitarxxx]

had my 90 out for its three monthly run.....temps about 50 F

noticed on full or eco power mode that tickover was steady erratic and would be between 900 and 1300....

but not quickly variable as in going up and down between these figures while stationary.....it wasnt pulsing.....but just while standing.... something like, start it one time and all was well at 900.....then stop at some lights.....would be 1300......then stop at another set....900.....and so on on

linkages? vaccums? .....l dunno

Dave