I run all my guns on a 7.4v LiPo and it's absolutely fine. Semi and auto depend on site rules. Lots of CQB places are semi only for example (not an issue if you're a woodland player though usually).
If you're concerned that the gun isn't working properly, take it back and get it replaced before you leave it too late. My M4 used to lock up on semi loads before I 1) fitted a mosfet and 2) learned to control the trigger spamming a bit. A lot of it comes as Samurai said from not pulling the trigger back fully and allowing the gearbox to complete a cycle before trying to fire again. If you want to fire a load of rounds, use auto as you say. That's what it's for after all!