If you weld for a long period of time without letting the machine cool down, you'll notice that it wont weld quite as well. It will seem to lose power and or penetration. However I've pushed this one to it's "limits" many, many times, and it's held up just fine.
I tried one of the new constant voltage (?) machines that Miller makes/made to replace this machine...........................it sucked.
As far as a technique for welding with this machine, I find that back stepping backhanded with a cresent movement of the gun works really well on heavier material. I also use a Tweco gun and hose assembly as the original was a POS.
I'm not sure but I think I linked a logsplitter (that's been stolen

) that I made in another post. That was welded up with the afore mentioned machine.
A 30 amp circuit is just fine.