1) I would pull out the pump and clean the impeller. What brand is the pump? Skimmer pumps are working in a truly terrible environment (mixed air and water) which is very corrosive to metal parts, and often the shaft is metal in larger skimmer pumps. I've had a large skimmer (Warner Marine, I think the brand is out of business now) with a rather pricey Sicce skimmer pump, and the pump only lasted 3 years. They certainly are not "lifetime" items, however 3 months is certainly a bit short!
2) I've never heard of SCA-301. However, I found it on amazon. Since the air line is entering the water flow before the pump, it is likely a needlewheel style skimmer. This means that the air is pulled into the water stream by the venturi effect, and then a specially modified impeller chops the bubbles up to increase surface area. This is in contrast to normal pump impellers, which instead look like a star (many flat fins joined to the central shaft). Therefore, they are not interchangeable with normal pumps. It is important to get a similarly sized pump, as the water level adjustment has a finite range and either too strong or too weak of a pump will result in the skimmer not working.
There are other styles of protein skimmers which use more traditional pumps, really inexpensive skimmers or very small units are often simple venturis, meaning the venturi is after the pump (no needlewheel chopping the bubbles); additionally there are jet skimmers (like the AquaC EV series) and in ye-olde times people used air-powered skimmers. But needlewheel skimmers are by far dominant in the 50g and up skimmers. Once you get to commercial skimmers other technologies are used, but that is beside the point.
