360 magnum ?

Check rockers....
Check lifters and cam...
If you didn't change anything then check your distributor springs and timing, and of course the idle-rpm.
Have you simultaneously lost power?

The ign timing at idle has advanced....for some reason. Stuck dist weights or dist moved.

This above, step 3. I’d also like to add, do a compression test. If you have the tool on hand, it’s a quick one to do. The tool is cheap and valuable. Get one if you don’t have it.

These are the quick and easy checks that should be done. The distributor is the easiest. Under the valve cover could uncover small or catastrophic issues.
Start with the simple stuff first.