I fly a flight from KLAX to KLAS with the aircraft I just purchased because it is a short flight and has various altitude and speed changes. The plan I use is OCEAN2 departure to FL250 and the KEPEC3.DAG arrival to RWY25R. Both the CS757 and CS767 have the same problem. The takeoff and departure is  flawless.  All the parts of the track and labels are shown in the HSI map.. In this arrival the A/C should be at 250KTS and 13000 at CLARR and hold that thru SKEBR and KEPEC than dec to 11000@230KTS for IPUMY, 9000 at NIPZO, 8000@210KTS at SUNST and KIMME, 8000@170KTS at CHIPZ, POKER, PRINO, than goes to ILS Lock and starts approach.
At TD the aircraft slows to descent speed and starts to descend, however the pink diamond showing the progress on the vertical deviation scale goes right to the top and the vertical descent and the A/C slows to about 1200 FPM. I do not reach 13000 till just before SKEBR and at 250 KTS. Between KEPEC and IPUMY it starts to descend but when it gets to 11000 the vert speed indicates a slight up and I start to gain altitude. The speed never changes as it should. At NIPZO I'm at 11500@250KTS, when the A/C reaches the way point  the vertical pointer drops to the bottom of the deviation scale and the A/C goes into a descent again and the speed bug and A/C jumps to 340 KTS. I let the plane continue to see what will happen. It stays at 340KTS and 8000 feet.  This has happened the exact same way for both models (757-200/767-300). In both cases the FMC had all the numbers correctly  for the altitude and speed changes. VNAV and LNAV remained engaged as did the AP. I flew this three times with the same results so I wanted to get feedback to find out if anyone else has had this problem.
I run FSX on a Custom build ACPI x64-based pc with an i7- 3930k chip, nvidia GForce GTX680. Windows 7. Nav data is CS1301v2 from navigraph. Cost index used for the flights were 100, 80, 50

Yes I set up the various speed constraints listed in the STAR. Since my post I tried the flight in reverse. I flew from LAS to LAX and everything went fine. I had the speed set up at 180 for particular waypoint and the model reacted fine. I had only one instance where the speed jumped to a higher speed. I do believe when the model goes to a steady altitude during an arrival the fmc reverts to cruise speed no matter what the alt is. I had the speed set to 240 on approach to lax rwy 25L. The alt between wypt LUVYN and Krain is 10000. It was at that time the model jumped to cruise speed (319). Maybe be an fmc glitch.