This will sound totally unrelated, but I can personally attest to it being related.

I never got the speedo jumping around, but I did have TOD lights flash and radical tach gauge jumping around.

For me it was all EGR leak related. The jumpy tach was due to the 22mm bolt under the EGR being loose. And the TOD lights flashing on came from air leaking from the connection/pipe that goes between the EGR and the intake.

The jumpy tach became so consistant at being inconsistant I could tell when my 22mm bolt had loosened again by my tach.

My suggestion: Check carefully and closely for any air leaks between the pipe from the exhaust header to the EGR and from the EGR to the intake.