Post SP1 not holding speed on VNAV climb

Today I started a flight with the 738WL Transavia livery from ENGM to EHAM. On the climb using CMDA and VNAV - LNAV above FL200 VNAV didn't regulate the speed and tried to achieve FL340 as soon as possible. The climbrate was 3400Ft/m and the speed just a little above stall speed.Now, was I doing something wrong (the I flew the 737 is how I usually fly the plane and I never experienced something like this) or is there something with the plane? Anyone else having this experience?Here's the flight plan so maybe someone like to try and reproduce the failure:FL340 SKI VES JUIST EEL SID: 19L TOR1C TRANS: 18R ART2CHope someone can help me out!Kind regards, Maarten