Ok, so I usually use the shutter in my lens, but recently I've been playing with enlarger lenses on my speed graphic. However, I've noticed... While speeds 30 and 50 always work perfectly, when I crank it up to 125, 250, 500 and 1000, the knob gets progressively more stiff, and when I fire the shutter, it sluggishly, slowly slides across at about 1/4 of a second... When I do this a bunch of times, it sometimes gets faster and seems to work properly, but then the same problem happens again. It's almost like it needs oil, but you can't just silicon spray it, can you?

Common problem. Sounds like it's time for a CLA! Are you mechanically inclined, 'cuz it ain't hard, but if you don't like fixing watches...
It means removing the camera back & upper + lower shutter mechanisms and cleaning them, then put 'em back together, wind the spring tension back up, etc.,etc.
Highly recomend you get a cheap reprint of the factory service manual if you've never worked on a Graflex shutter before.
Other alternative is to ask Les. He's fixed more Graphics and Graflexes then any of us will see in a lifetime!