Sounds like same compressor as one of mine except mine has a 20 gallon horizontal tank... It has a beautiful gold "5 HP" plaque on it but is about 3.1 real HP. The motor HP plate just says "special". Like yours it's from back when the Govt was allowing lies about air compressor HP. Over the years I've seen actual 2 HP compressors called 2 1/4, 2 1/2, 3, 3 1/2, 4, 4 1/2, 5, 6, 7, even as high as 7 1/2 HP before the semi-crackdown! Of course anything over a real 2 HP would melt down a 120 volt plug/outlet. Shop vacs were also dishonestly labeled. Higher HP rating = higher price. Mine's black because I bought it 30-35 years ago as a 'Rocket' brand which is a rebuilt Samborn. It's 2 cylinder single stage. It's single stage because both cylinders are the same size and quickly do the same thing in parallel to 120 psi. IF it were dual stage it would likely have one large cylinder that compresses the air to a certain pressure and then a smaller cylinder that compresses the air to a higher pressure like 175 psi. It also would likely be a real 5 HP and be hard wired rather than plugged in.
About 3 or 4 months ago mine would only go to about 60 psi and then the belt would start making squealing noises and a bad smell as it was burning up as it started slipping. I thought the motor adjustment slots were all used up but discovered by loosening a shield holding bracket that was in the way I could give it one last tightening of the belt. When it starts slipping again, I either get a new flat belt or lengthen the adjustment slots. I think the belt does still slip momentarily from motor high starting torque when it's first turned on.
Your outlet valve(s) to the tank could be leaking back into the cylinders, this would prevent building up above a certain amount of pressure because more air couldn't get into the system, but that shouldn't really cause the motor to struggle, it just wouldn't make enough pressure. The motor should be able to handle the lighter load though as the lower pressure air moves back and forth.
