1999 2.0ltr erratic idle

[welshman61]

I have had this car for approx 3 months now and it has always had an erratic idle.
It always starts first time and idles evenly for about 30 seconds then gradually starts to become lumpy and continues to run rough up to approx 1500rpm.Above 1500rpm it runs fine.This happens everytime it is started and also when the engine is a normal temperature.
The management light has never come on,which I find strange as i assume it should detect a missfire and put the light on.
I have stripped and cleaned the stepper and also the throttle body and changed the plugs, but no difference.
I have put a dignostic reader on and it finds no faults.
I am getting a bit confused with some other posts.i.e coolant temp switch being a possible cause.
I think my next step is a new coil pack,then maybe a new car lol
Well at least I have found out that an engine mount is worn as it bangs like hell on tickover !!
Any thoughts on this?
I have seen other similar problems but not quite the same as mine so maybe it might be something else?

[highlander]

Throttle position sensor - maybe replace it?
EGR valve might need cleaned?

These are things I've had suggested to me; I've got a 2.2 petrol Coupe and it does the same thing. I haven't cleaned the throttle body yet; will try and have a go this weekend.