2008 Mac Pro only restarts in Recovery Mode

My 2008 Mac Pro has been running Mavericks 10.9.5 happily since last year.However, in the last month, when I restart, it automatically boots in Recovery mode and continues to do so if restarted.

- I have tried resetting the NVRAM multiple times and this has no effect

- I am unable to boot in Safe mode

- If I run a disk repair in recovery mode, it tells me everything is fine.

- Re-installing everything from Time Machine has no effect.

- The date and time on the machine are still fine, so I think the PRAM is OK.

The only thing that works is to re-install a new copy of Mavericks. This has now happened twice.

Mac Pro, Early 2008

Processor 2 x 2.8 GHz Quad-Core Intel Xeon

Memory 8 GB 800 MHz DDR2 FB-DIMM

Graphics NVIDIA GeForce GTX 660 2048 MB (I replaced the old video card several months ago, but this boot problem only started about a month after the card replacement, so I doubt it is the culprit.

Software OS X 10.9.5 (13F34)

Help!