AX Idle speed vascillation

My wifes M reg 1.0 AX is driving us nuts. At idle, the engine speeds up and down once per second, from about 1300rpm to 800rpm. Just as if I was
blipping the accelerator pedal. Curiously, when cold, it starts up fine and runs at a fast idle. Then after 20 seconds or so the speed down/ up starts. When driving along, every so often the engine hestitates and bucks, but resumes its relativley composed nature. When cruising with foot off throttle and in gear, the car runs at a fast idle (25 in 4th). Its only in the no load situation when it does the revving up/down bit.
Got an autoelectrician to look at it. No error codes in ecu. He said it might be a throttle postion sensor. But I then removed all the sensors I could find - water temp, lambda. No difference. The AX has a Bosch ecu, but I have swapped out today in desperation with a reclaimed one (cost £50) and it still has the problem.
Any ideas anyone?
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