Revs randomly going up and down

[Oni]

Hi all,
Once again im hoping someone here can come to my rescue. I don't have a lot of info i was surprised then the below happened but its happened once before. I had been driving for about 5 min in stop and go trafic then had to merge onto the highway, so i accelerated and changed into 4th at about 4k rpm. that took me too about 3.5k rpm in forth gear. under acceleration the needle then started jumping around between say 3k rpm and 4.5k. i backed off and it stabilized and i wasn't able to reproduce it.Sorry if its posted before i wasnt able to get the search working ( a 430 error ).
Ill try get some more info if i can and post back again soon. Some reading said that it might be a bad ground. Im pretty new to cars i just learned to change my oil so any Noob friendly troubleshooting tips would be great oh and the car is a 1990 manual mx-5 with 190,000 k's.

thanks heaps

[daffyflyer]

Are the actual revs changing (which would make you lurch horribly) or is it just the tacho going nuts?

[bruce]

Plug leads.

[Oni]

Hey thanks for the replies, its just the tacho. while its doing it it still accelerates smoothly and doesn't behave any differently

[daffyflyer]

Mine does this too, but not as bad.. NA tachos seem to be the worst bit of the entire car

[Big G]

A lot of rev counter problems are caused by issues around the coil pack, the drive for the tacho is derived from the coil pack and a loose connection is often the cause