Hmmm. A local fly shop would probably be a HUGE help here to narrow it down and give you some pointers.
Otherwise, I'd try looking very closely at the surface to see if you can find anything floating or swimming up, back water eddies that might collect stuff or spider webs by the stream, to see if there's anything caught, and get low to the water to see if you can find a mating swarm of something tiny flying just above the surface.
It sounds like something is either popping to the surface very quickly, like a caddis, or there is a mating swarm of something small and tiny like midges just above the surface.
Whatever it is, it's either very small (or you'd see it) or if it's a larger size, it's not sitting long on the surface (they wouldn't be jumping).
Based on that,
try swinging a wet. Something like a size 14 Green Emergent Sparkle Pupa is usually the first out of the box for me if i think it might be a caddis. Sometimes the emergers rocket to the surface, pop off and fly away. There are many caddis that hang in faster water, usually below riffles, and this is a good pattern to imitate them. But there also some slower water ones like the Weedy Water Caddis. If you look around you might see their cases-- they look like little ice cream cones. Another caddis case you might see are ones that look like little turtle shells (saddlecase caddis) but they're typically found in faster water below riffles. But a Green or Tan pattern and size 14 imitates a lot of different caddis, including these. If you wanted to try a dry fly, a 14-16 black elk hair caddis (weedy water) or tan 16-18 (for saddlecase) might be worth a shot. Swinging a soft hackle (Partridge and Orange/Green/Yellow, Partridge and Peacock) can also be effective since these caddis, and several others, come back to dive under water to lay eggs and can double for the emerging pupae as well as drowned adults if you have something like that.
If you think they might be midges flying just above above the water that's got them jumping, something like a Griffiths Gnat 20 or smaller, or anything you've got that's tiny- small Adams, BWO, caddis or midge dry etc on a fine tippet would be the way to go. Usually, if midges are emerging or actually on the surface, it's more of a subtle, sipping rise-- after all it's a small meal that doesn't really justify a lot of effort as far as jumping. But if they're swarming over the surface out of reach it might be to much for them to resist. Dangle a tic tac over my head long enough and I'd jump for it eventually
Other stuff that they could be on are Pale Morning Duns and Callibaetis- and they can be a very tricky hatches and are major ones out your way, also in slow water sections and lakes for callibaetis. But it doesn't sound like that. Again, more of a sipping deal, since the emergers don't usually pop off, and the duns are usually on the water or stuck in the film and are pretty easy pickings.
Hope you can figure this one out, good luck and let us know.