Bumping an old thread as I`ve just had a similar issue with my Z.
Really strange, on M4 at the time, wipers were on intermittant and had sidelights on.
Wipers went to full speed, headlights came on and rev counter died. Stability,traction and check engine lamps came on then wipers stopped. 3 odd miles to next exit.
How i laughed.
Anyway, using manual selfcheck/diagnoses guide did the cars self test and everything worked fine. Ended the test, disconnected battery, pulled all the plastic trim and had a poke about on fusebox.
Put all back together, reconnecting battery and started fine.
Been fine since although check engine lit, assume fault code is registered with ECU.
Any strange behaviour with either Lights, dash displays, wipers, heated rear screen,radiator fan or AC clutch seem to point to the IPDM.
Further investigation ensues, anybody enlighten me further be much appreciated my cars rarely fix themselves?