Interesting.... I too am having this issue. It just happened out of the blue as well. I replaced my engine mount a year or so ago but I guess it's possible it could have issues again. It could even be the trans mount. I've felt a dp hit the sway before after I put in headers. You could definitely feel it before i flipped my sway. It vibrated the floorboards terrible. I'm curious to see what you find out. I'm gonna give my mounts a once over when I get a chance.
Although my issue seems to happen if I get into any boost above 2psi instant kr out tha wazoooo!
