My trusty S40 2.4i (93000km on the clock) started doing some weird things a few hundred km's back - as I would approach a stop street or traffic light, the revs would drop way down to a few hundred RPM, and sometimes the car would even cut out completely.

I would then restart the engine and rev it up a bit, and I could carry on normally.
Then it really got bad last week, to the point that the revs were fluctuating between 200 RPM and 1500 RPM - I couldnt even pull away.
Right about then, the engine warning light came on and said "REDUCED ENGINE PERFORMANCE - ENGINE SERVICE REQUIRED..." or something along those lines.

I limped to a local garage (family friend), and he plugged in a diagnostic program which immediately said that the fault was coming from something called the "Throttle Valve Control Unit" - apparently quite an expensive little part.

In an attempt to try and do a quick fix, he unplugged the connector leading into the device and sprayed some cleaner / contact spray onto the contacts.
He then reset the fault on the computer, and sent me on my way without too much confidence that it would help for very long...

One week later and the car is still running super smooth and without any of the weird symptoms it had before.
I will report back here if the issue recurs, but hopefully this little fix is a money saver for me (and anyone else out there who may have a similar problem!)

 

2007 s40. NOt sure if my issue is similar. For no obvious reason while driving wipers come on automatically and then alarm messages and alarm inidcators all come on and then immobilizer message comes on and engine looses power and then turns off.
Volvo dealer changed alternator but issue happened again. Had car at dealer in failed state but volvo diagnostic software couldn't diagnose a problem . Now out of ideas.

 

Congrats to the OP for fixing the issue. I watched a youtube video where the poster removed his Electronic Throttle Control (aka drive-by-wire) and cleaned it up with throttle body cleaner and replaced it with a new gasket which fixed his erratic idle.

For the hijacker () it sounds like you have a loose connection on either the power cable or ground cable or maybe even a bad battery. I've seen internally shorted batteries do weird things.

 

@Hudini funny one about the hijacker
I live in JHB, South Africa so that's commonplace here

Jokes aside though, that is exactly what an anti-hijack system does... is there one installed in the car by any chance?

 

Anti-theft alarm is about it. Far as I know. Is that what you mean? Because we have something called a "car jacking" which means someone forces you out of your running car at a stop light or parking lot.

The anti-theft alarm is built in from the factory. When you lock your car the alarm automatically is set.