Radio interference is something that even stymies the professionals more often than they care to admit, I'm sure.

Without some pretty fancy and expensive equipment, you can't hope to even begin diagnosing the root cause scientifically. The best you can do is change things one at a time until the interference goes away.

The 400 motor that comes with the Easy Star should have capcitors built in. Adding capacitors won't hurt, though.

If rearranging components doesn't help, try resoldering the connections. Replace one component at a time if it comes to that.

Just remember to use trial and error, making sure to only change one thing at a time before re-testing.