The burner is failing, this is a common first symptom in that model series. The 310 is nine years old, far beyond the typical life expectancy of a DVD recorder on its original burner.
It is repairable by replacing the DVD burner, but at this point may not be cost effective. You would need to track down a still-fully-functional Pioneer DVR-106 generic burner made for PCs. Most of these are worn out but now and then you find one on eBay that wasn't used much. They cost about $30, give or take. You also would need to get the special Pioneer/Sony Service Remote
, which is another $30, and then contact the volunteers at pioneerfaq website to make a donation for help in getting the Pioneer GGV-1256 Type 1 Service Data/ID Disc. Once you have the replacement burner and the service tools, you would remove the old burner from your 310, open it up, and exchange the controller circuit board with the one in your replacement burner. Put the replacement back in your 310, and follow the instructions at pioneerfaq to reset your 310 with the service tools so it will accept the new burner.
I've been using/repairing/upgrading all models of Pioneer DVD recorder since 2005, I love Pioneer recorders, but IMHO at this late date its probably not worth the effort and expense to fix a 310. All the DVR-106 replacement burners are quite old now, and will likely fail shortly after installation, leaving you stuck again. I'd recommend you start putting some money aside toward a new Magnavox MDR-533H DVD recorder with HDD and modern digital tuner. These are easier to use than a 310 and the burners are very durable. The 533 should be available at WalMart website this fall for approx $250. If your budget won't stretch that far, you could consider the Toshiba DR430
at about $99 (this is the Magnavox without the HDD or tuner).