（Domo） Arigatou Gozaimashita (どうも）ありがとうございました　is in the past tense all right. But when you say thank you automatically it is a past tense. Don't you think so? Grammatically speaking you're suppose to say ありがとうございました but there's no problem in saying ありがとうございます　either. Oh, by the way, sometimes I only use domo （どうも~）.

grammatically, the second one is in the past tense. I don't know if it has a subtly different meaning or not.