I have an 04 S60. Twice I had this problem. I had a thread the first time (scarry problem) without response. The problem came back and I have new information.
The car downshifts a gear suddenly. This time I saw the DIM (drivers information module) act up. All lights went on and the speedo and tach dropped to zero. It acted as if the DIM rebooted. This happened several times a few minutes apart. I got an ABS warning message. At a stop light the temp gauge went to zero and back to normal when I accelerated. Later the DIM went dark. Nothing worked. Accelerating from a stop, it acts like I am starting in second gear. The funny thing is the windows also would not work or the mirrors or door locks. If it wasn't for this, I would assume it was the DIM. Like last time, I turn off the car and restart and it all seems to have gone away. Is this a DIM problem? What other module would cause these problems? The last time I thought it was a battery connection problem and I cleaned the posts. It worked for 3 months without problems.

 

You have issues on both CAN networks Hi speed and Low speed.
So the faulty module is probably CEM.
Do you hear any clicks at the time it happens?

 

Clicks? That is really hard to say. My hearing is not all that good and with traffic etc. I can't say. Can you tell me where the CEM (Central Electrical Module?) is located? Does it need programing? If I find a bone yard one, will I be able to use it?

 

A little additional information. After the first time this happened, The SRS airbag urgent message came up. I wrote this off as another random problem with all the other issues. I have not taken it in to the dealer (sticker shock fear). I think only they can reset that code.

 

Look here Topic with CEM removal insructions

Used CEM wont work for you and also cannot be programmed by dealer.
You can try to ask Xemodex for rebuild.

BUt before a would get errors codes scan and post here. Better do it at the dealer or try to find a guy with VIDA + DiCE.

 

I have a simple OBDII unit. The only codes I had were for emissions. Do I need a different device? The check engine light is currently out. I do have an SRS airbag service urgent light. This came on after the first incident. I assumed it was a fluke like the ABS warning.

 

As I said you need VIDA + DiCE.
VIDA is software, DiCE is device.
SRS can be registered because of can bus issues but we need to know the error codes stored.
Ask local dealer how much the take for the errors reading.

 

Double bug. Thanks for the reply. I Will look into having the readings made.