RPM reving going down hill????

The RPM's in my 92 7.4 rev up and down as I go down hill. I have replaced the ignition modual and put new ignition wire on it but it still does it. It does not sound good. Thanks in advance.

FYI, My '91 7.4 started doing exactly the same thing when I replaced the stock exhaust system with a custom dual system (H-pipe, flowmasters, low-restriction cats). Been several years now & it still does it. A bit annoying, but doesn't seem to have any otherwise negative effect.

possible coil issue? Throttle position sensor?