Something we had happen with my car.
When we installed the new computer we didn't run the power to the coil through the tacho as per the factory wiring. The car would get to around 4,000rpm and the timing would go haywire causing it to start missfiring. We spent a long time chasing it and in the end found it was because we had used the power supply from the computer to power the coil instead of the factory power supply which runs through the tacho. We changed it back to the factory Z power supply and it's been fine since. There is something in the tacho that takes away the electrical interference we were experiencing.
Anyway, I hope whatever it is isn't too hard to sort out