Flightplan disappearing

Does anyone know why the flightplan in the ND shows my departing airport to
my destination in a straight line only? What I'm saying is, I'll create a
flightplan with waypoints into the CDU. Execute and saves fine. Then, in my
ND display, I'll have the created flightplan visible fine. I'll then proceed
to take-off, fly about 4 minutes, and then the flightplan turns into a
straight line from A airport to B airport. Then after about a minute the full
flightplan appears again. This happens continuously, back and forth.
Obviously I have a problem. I'm using all newest bulds. File check passes fine.
Any ideas?

Thanks

Kev Stefan

I had similar problem.
After flying a plan from EDDC to EDDF I entered a new route
manualy into the CDU. A small tour around Frankfurt airport.
I execuded the plan and I could see in the PFD.
After a while, suddenly the plan disappeared. I pushed EXEC again and the
plan was back for a few time and disappeared again. Always when I push EXEC it
comes back.

I use latest update of CDU, PFD and MCP

Cheers

Thomas