Been trying to fix this on my 850 for a while, thought it would be the engine coolant temp sensor, but alas, just replaced it and didn't change a thing
The car idles at around 2000rpm after a cold start, slowly climbing up to 2100 or 2200rpm for 10-20 seconds, then starts alternating between 1000 and 1500rpm, at 0.5-1 second intervals. It doesn't go up and down "smoothly", like a sine wave kinda thing, it's more like a switch, up, down, up, down. A square wave if you will
Could be related or not, but a few months back, the car started dying randomly while idling after a long-ish drive. Didn't happen very often, but every now and then, I'd be idling waiting for someone, and it would stall. It would then have trouble restarting, spluttering and throwing a little fit for a few seconds until it would eventually start like nothing had ever happened.
Any thoughts would be really greatly appreciated! I've replaced the ECT sensor and the thermostat (old parts seemed fine, and as I said, the problem hasn't changed in the slightest!). I've got spark plugs, dizzy cap and rotor ready to swap in, but they aren't that old and I'd rather not replace them unless there's a slight chance that they could be part of the problem (which doesn't seem very likely?!)
Oh, strangely enough, no codes at all..