When my car slows right down the engine revs drop suddenly, I can see the tacho reading briefly dip towards zero, as if it might stall. Then it recovers back to normal 600 rpm idle. Auto box has been serviced recently so that should be o.k. Could it be air leak or electrical problem?
Has this only started since the transmission service. If so what ATF was used? Other than that diagnosis would need to be on the car rather than on this site. This is to separate an engine from box problem