"I had some mood swings," Megadeth frontman Dave Mustaine said in an interview about his recent performance with old bandmates Metallica. "There were some ups and downs and stuff. And, you know, got excited, and kind of got impatient. 'Let's go. I'm OK. Well, let's go!' and this kind of thing and that's just the artist in me. I'm just squirrely like that."

Mustaine joined Metallica on stage this past weekend to celebrate the band's 30th anniversary. The historic event went down on Dec. 10 at the Fillmore in San Francisco, Calif. Other musicians joining in on the festivities included former Metallica members Ron McGovney, Jason Newsted, Lloyd Grant and Hugh Tanner. Original Black Sabbath singer Ozzy Osbourne and bassist Geezer Butler also showed up for the monumental jam.

Mustaine was a member of Metallica from 1981-1983 until he was fired for alcoholism. It was after he split with Metallica that he formed Megadeth.

Most metal fans realize the significance of Mustaine and Metallica joining forces on stage again after nearly three decades. Mustaine admits he just had to let himself go in the situation.

Watch Megadeth's 'Wake Up Dead' Video

"I wish I would have had a little bit more opportunity to get prepared with the band. You know, because I'm a perfectionist... I would have liked to have had my sound just so and make sure when I did the solos they would jump up the volume and stuff like that that I'm used to, but we were at a club and playing like a club band. It was fun to take off the rules and regulations and stuff and kind of shoot from the hip."

Mustaine and the rest of Megadeth will hit the road this January, once again heading up the Gigantour. The tour will also feature Motorhead, Volbeat and Lacuna Coil.