A well serviced TS 4cyl..........started missing at idle a couple of weeks ago, will drop from 850RPM down to 550RPM back up again, down again etc, no loss of power and heaps of acceleration (for a four cylinder) cruises beautifully without missing a beat.
The plugs, dissie cap and rotor are new, the leads about a year old and injectors are about 18 months old, no vacuum leaks, coil about two years old as is the fuel filter.
It feels like a cylinder is dropping out, plugs are normal colour, and I've even reset the ECU.
Doesn't burn oil, in fact still doesn't even use oil, but the chains rattle a bit.
Any ideas???

when the car is idling do you feel anything when sitting in the driver seat?
when crusing along and you put your foot down does it take awhile for the car to respond before dropping a gear then taking off?

the only thing you feel in the drivers seat is the vibration from the revs dropping, when cruising the acceleration responds immediately, it's a manual btw, so no kick down.

hmmm the only thing i can think of at the moment is the spark plugs, but you've said that they have been changed and you have already checked them.

i'm going to put my money on the idle controllers

are the revs dropping when the fan turns on? when idling listen for the radiator fan. if it starts up and you lose revs while its running, i wouldnt lose sleep over that.
how long since a timing belt replacement? it could be slipping, or it could be the fuel pump. check things in that order and listen to the engine when its hot and idling, you should be able to hear where the noise is comming from.

i'm going to put my money on the idle controllers
Where are they?

hmmm the only thing i can think of at the moment is the spark plugs, but you've said that they have been changed and you have already checked them.

Just because they're new doesn't mean they're gapped correctly. This was an issue I've had in the past. The OP might want to check that out.