I like to run long flights in the 767 with PAX and cargo totaling around 34,400 lbs and CI at 94.  At full fuel load I experience pitching + and - 1,000 ft at cruise altitude FL290.  The only way I could reduce the pitching was to reduce the fuel load, but then twice I've had the "insufficient fuel" warning on the CDU for flights over 4500nm.  My workaround (some would say cheat) was when pitch at cruise altitude stabilized at around FL3500 to open the fuel and payload dropdown and add more fuel.  I haven't done it systematically to check if that causes  the cruise range to fall in the ranges Boeing claims, but I'll do so in the future.