I'm justsaying I got my car checked out before I bought it and the mechanic said it was good. 3 weeks after, I blew the engine up. I went WOT at 80 MPH, and coolant leaked into the engine, so I guess you could say it happens only at "high speeds". AND just because you see it at high speeds doesnt meant it isnt causing problems...
Get a new air filter, oil change, oil filter, spark plugs, wires, and coolant flush (just cause) and see if the problem is still there.