FMC waypoint speeds reduced as we progress.

HI,

I am currently flying from EGLL to OBBI and since departing I've had to battle an issue where the next 1 or 2 (sometimes more) waypoints have their speeds set to 170kts in the FMC. I am currently at FL350 and have set the MCP speed to .83mach. If I leave it on FMC speed, the aircraft slows down to the slowest possible safe speed, just above stall as the FMC sets the speed to 170kts.

Now, this happens as the flights progresses. For example, in the FMC I can see that my waypoints currently are:

BOREP 170/FL369

BAMAS 170/FL381

KEPOM .828/FL390

Now, when the aircraft reaches BOREP, KEPOM will be altered to read 170/390.

Now, there was one occasion during this flight that the FMC did say .831 for the next waypoint, however, when I flipped back to FMC speed from MCP speed, the aircraft still slowed down to almost stall speed, regardless of what the FMC speed was.

I have read the introduction manual and have been through the FCTM and FCOM 1&2.

If it helps, I'm flying with a cost index of 72.

Any ideas?

Rynardt