What kind of SD card are you using?
When I first bought my R-09 back in 2007 I did some serious in-home testing of it and found some serious issues with the SD card I had bought to use in it (and even the same brand replacement SD cards after the first one). Now I only use one brand and I have 8 of them that have never failed. I won't mention the brand names to bias anything right now.
I always use Energizer Lithium AA's. I use to use rechargeables back in the DAT deck era, but they can fail at just about anytime even when they seem fine (i.e. they can all of the sudden drop off when you least want it). For me the price of the lithiums isn't an issue for the security they provide, especially when it's a recording you have only one chance at.
I would buy a pair of AA lithiums, put them in the deck, set it up in front of the TV and hit record and get some levels going, set it on hold and let it record and see what happens. If the deck shuts down at anytime before the SD card is full, then I'd definitely check the SD card, try another brand if you have another one.
That's how I did my original R-09 testing and the "bad" SD cards were doing exactly what you are saying. I'd come back anywhere from 15 minutes to 1 hour after recording was started and the R-09 was shut-down (time depending on which SD card I was trying). To double check it wasn't a bad battery, I plugged in the power adapter and did the same test again. And again the deck was shutting down and I could for sure say the batteries were fine.
Since I switched to the SD brand I have used since then I have had zero issues like that.