90 Accord running Rough

My 90 Accord starting getting a vibration at around 45mph or 1800rpm. It is some what like the feeling you get from a spark plug wire breaking down. I can also get it to hit this rought spot by putting the car in drive and holding my foot on the brake and raising the rpm, by putting the car in neutral or park and raising the rpm, with or without the a/c on. Once you go above or below that range, it smooths out and runs fine. I did put a new set of plugs in it today with little if any change. Any ideas.

Thanks,
Rodney