On my 1998 2.1TD, I had a similar problem. It was a small black box right up behind the glovebox. It cost about ?40. Mine gradually got worse over time.
2007 Mondeo Titanium X 2.0 TDCi
2007 307cc Sport 2.0 HDi 136
Gone but never forgotten: 2002 406 Coupe SE 2.2 HDi
My speedo drops down during some right hand bends, but corrects itself again afterwards.
I disconnected the ABS pump before and the speedo didnt work at all, so its included in the equation somewhere as well.
Check all the connections from the transponder (somewhere on the gearbox), the ABS pump, and also this wee black box? that neildavies is on about, and possibly the connections on the back of the dash unit as well.
French electrical systems are renowned for their reliability---NOT!!
9 times out of 10 though will just be a connection dirty, poor fitting or completely pulled out of position.
The actual electronics themselves, most of the time, wont be to blame, as even French modern electronics are very reliable.
If I ever get my hands on a haynes manual, or wiring diagram, I should be able to solve all of your electrical problems. Its my forte!
Actualy most 406's had there wiring looms made in the UK (Worksop), several costly recalls for a 8" length of 5mm cable (so iv been told).
Touch wood thou mine seem to be ok except for remote that works itself, A/C not working, and 2 loud bangs from behind dash on seperate occasions (wiring for both tweeters blown). Well i will have my music loud.
Well i will have my music loud.