DDR HPCII calibration successful but then only works briefly

I am trying to run DDR DRAM using an HPCII. I have Quartus II V10.0 SP1, and a Cyclone III FPGA. My DRAM initialises, calibration is successful and local_ready goes high, both in simulation and on the real board. If I then start to use the DRAM controller to write to the DRAM then the simulation runs fine and I can see writes to the DRAM model. However, on the real board, I get a few hundred or a few thousand writes and then local_ready from the DRAM controller stays low. This happens even if I use constant value address and data. If I vary the address then the length of time the controller works before local_ready sticks low seems to depend on what sequence of addresses I use. For some sequences of addresses I continue to get local_refresh_ack going reqularly every 7us or so, and I see RAS and CAS being generated to the DRAM on the board. But in all cases I can't use the DRAM any more as local_ready stays low.