My favorite run is the beginning of the second solo in "Good Times, Bad Times" by Led Zeppelin, which is pure descending pentatonic scale abandoned at the right moment. I'd say you're talking about something like the opening solo for the 2nd part of "Sweet Child of Mine", which is pure Slash going crazy. I've had the same problem at some point, disability to play fast runs longer than a few seconds. What pulled me out was not the mental ability to construct great runs I could play, but the physical ability to play whatever crashes into my head. Try the following (in terms of moving through 3D space): walk around, run down a slope, jump over a canal with one leg and back with another following it's direction, run to the edge of a cliff and dive into the sea, paraglide back up just before you hit the watter...
Enough. I think I need a walk in a park or something. Woof! Woof!