MART 182

Hi

I have a 1993 Fiesta Flight which has an idling problem.Seems OK when cold starting idling at 900 - 1000 rpm then drops to 800 rpm when warmed up.
Then the rev counter needle drops below 800 rpm every 10 seconds or so and the engine shudders slightly.Revving the engine very gently will produce the effect and revs drop more then climb back up.Whilst fiddling about trying to establish what was going wrong I discovered that if I operated the electric windows and kept my finger on the button the revs really dropped almost to the point of stalling.Maybe the problem is electrical ?
I've recently had the car tuned and sensors cleaned up in the carb and running speed wise it's excellent for it's age.
Appreciate it if anyone has an ideas of what could be the problem.

Thanks
Mart 182