It sounds like the same problem I had with my 2005 Yamaha Vstar. Come to find out it was a grounding problem on my tail light. I looked through the schematics for my bike and found out that every light on the bike is connected to one ground wire. I fix that situation by grounding every light socket that I had. That would be a good place to check because your speedo did a sweep and your tail lights dim light blink. That sounds like it's a electrical issue or a ground problem. Hope this helps.
Same situation I check everything electrical except for the ground.