Hi, I have a similar problem with the 54 mk2. My problem occurs at idle more so than at any other setting. My rpm reading on the data terminal reads 2600 without the engine running. It didn't start doing that until I put the engine on a Y/A Stingray and used the green cowl/inlet. Do you suppose I'm having issues with magnetism as well. Also noticed that it only registers a reading when a servo appears to be under load, ie; srevo noise. When the load is taken off the servo, the data terminal indicates 0 rpms. What type magnet are you using to de-magnetize. I'm also running standard noise traps on all long servo leads.
Steve