My 2 cents worth. Yes the spring could be getting weak. If so take some length out of it by recutting it and putting back under the screw. Easy enough to do or find one probably in Lowes or Home Depot.
Second the arm bushing could become dry and wont allow great movement. Take spring off and feel the tension on the bearing. Should be easliy moved up and down and if not then that bearings may need to be replaced.
Third thing is and this could lead to your vibration thing too, is that lever handle has a tendency to float over toward the side of the arm and rubs on it. This adds resistance to that arm lifting. Even though it is metal on metal it still can wear. I will usually get a metal noise hard to describe when that happens but know what it is so just move over again and is good for many many hours again. As I said many times here before I hardly ever play with that lever and never had to replace and my saw is as old. If I change blades to a larger blade I will just nip some of the top of the blade and keep same setting. Only do I touch that lever is when I switch to very fine blades such as Jewelers blades.
Also the vibration thing always check the obvious. Make sure the table nuts and bolts have not come loose. Make sure the 4 legs are sitting firmly and splayed properly. Make sure there is no small pieces of wood caught under the arms or build up of sawdust around moving points. Make sure all things are tight. Vibration can be transmitted many different ways and tough to locate. But a thorough check of things eliminates things. The people at RBI will tell you to do these things first off.