I had some porpoising issues on my flight in the LU -700 at cruise with LNAV and VNAV engaged. Cycling the AP fixed the issue temporarily, but then issue returned several times within my 2 hour flight. The -800 did not have this problem FYI, and I did a 2 hour leg in it. Couple other sim pilots experienced porpoising in the -700 in a Facebook post as well.