1991 Volvo 740 Engine idle

I bought a 1991 740GL Volvo recently. The car drove well for 3 days. But as I started the car the next morning, it would turn over easily enough but stall very shortly after. I could prevent it stalling by putting it into 'drive' quickly after starting, however the engine seemed to be running at a high idle and whenever I braked to slow down or stop the transmission would kick down into a lower gear with a clunk as the engine was running to at too high revs to allow a smooth shift down. When I paced the enigne in neutral the car idle would vary between 900rpm and 1400rpm.
Any ideas as to diagnosis would help alot.
Thanks Bart