My new Droid Razr, and Razr Maxx phones put a small delay in between each ring. Some of the factory ringtones are designed to be a continous loop. for example "medieval jaunt", "romancing the tone", and "Crayon Rock" are designed to be looped with no delay, and sounds odd with the short delay in between rings. I saw I similar question from about a year ago, which there seem to be no answer, but that was someone with the opposite issue, of wanting a delay. Thanks!
Assuming you have one specific ringtone picked out: take the audio file and loop it yourself (using something simple like audacity) and make it the length of a ring (typically 30 seconds).