couple of problems with my 400e
 

I just got a 400e and replaced every fluid in the car. I noticed today when I was driving the RPM was 2200 and all of a sudden it went to 4500 and back to 2200. I am pretty sure the fluids are ok but I will check.

also fuel guage is irratic. THe level sender?

Idle air control solenoid, is it OK? Just wondering

Love the 400E!!! They are super good cars, perfectly balanced, arguably the best setup for the W124 sedans, not counting the over the top 500E

Could be a bad gauge but probably the sending unit - not hard to replace yourself, in the trunk behind the back cover. Don't think there is such a thing as an idle air control solenoid on a 400E. But there is a 38 pin diagnostic connector from which you can read the fault codes.

I did some more research and many seem to have the fuel problem so I am leaning towards the sender unit in the back. I will have a look at that and replace it if necessary. I am not sure on the tranny though. I checked fluids again today and they are right in the middle of the 2 bars. I ran the car for a few miles and shifted through all the gears and checked the level. I will see if it happens again.