We have a 2007 Mazda 3 and I have just swapped out the battery now it occasionally (ie once or twice over 30 minutes) cuts out for no real reason.

When I watch the revs I can see the dial hovering around 400/700 rpm, going up and down in around 100rpm increments before stalling. On other occasions it sits happily at 1200 (from cold) or 900 when warm.

I have read a few things online about leaving it running for x minutes then doing y (such as leave it for 15 minutes on idle or 2 minutes on idle then switch on air con and then leave for another 2 minutes). Other Googles suggest I need to do a manual clean out the throttle assembly or just leave it as it will settle down when the computer relearns itself (it has been about 3 weeks now so it doesn’t appear to be settling down).

Has anyone got any other ideas/suggestions etc. Thank you.

How did the previous battery die?

I can’t help, but I did laugh at swapped out the battery, maybe if you just replaced it it would be fine 🙂

Alternator?
[video]https://www.youtube.com/watch?v=sBktUi4osF8[/video]

How did the previous battery die?

Old age – it was losing charge overnight.

That sounds more like an alternator issue. Too late now, I guess, to test the old battery, but you’ve can test the alternator quite easily – have a look on Google/YouTube.

Did it stop the battery drain?

Further reading:
http://singletrackworld.com/forum/topic/motorbike-battery-drain-what-to-look-for

(I was going with “bad earth” myself)

I imagine your cars got an electronic throttle body in which case it needs to relearn itself after disconnecting the battery, this is certainly the case with Suzuki. Disconnect the battery again & leave it disconnected for at least 60 seconds, then reconnect the battery and turn on the ignition – don’t start the engine at this stage, leave the ignition on for at least 60 seconds while the throttle valve goes through its calibration procedure, then start the engine.
If you still have idle issues you’ll need to clean out the throttle body with carb cleaner and an old tooth brush as carbon build up on the edges of the butterfly and in the throttle body prevent the valve relearning the idle position. You’ll need to disconnect the battery again and go through the 60 second learn process after cleaning the throttle body.