The rpm signal comes from the coil pack, I would say it could be a fault there or the wiring from it. The coil pack needs a good connection to the body to work properly, check it is clean behind the bolts and on tightly.
The later coil pack you got, if its from another EP82 the only difference should be a locating spud on the outside of the connection will be in the wrong place, cut it off if it is, it will do the same job.
I have seen funny tacho readings and bad idle from a poorly earthed coil before. Hope this helps you out.