That's a pretty good looking ST-8. I bought mine new back in the late 70's or early 80's, and it still works great. What cartridge are you using? These super-low-mass tonearms need a very high compliance cartridge. I'm using a NOS Sonus Gold Blue, compliance of 55, and it sounds great. I have a Soundsmith Otello, high-compliance option, inbound, compliance of 28.
Do you have any spare tonearms available? I'd like to be able to swap out different cartridges without mounting and unmounting them each time. Using the cartridge alignment tool is nerve-wracking.
There's a good thread on here about the alignment tool. Lots of other info in that thread beyond just the tool.