Which BlackBerry do you have? How old is it? Has it gotten wet? the sensor under the keypad / keyboard likely needs cleaning or replacing.
You may be able to get it replaced under warranty if it is less than a year old; other wise you may be eligible for a discounted phone. You can also look for a replacement on eBay, from a friend, or a CPO device.
I also have the Bold 9650 - there is a moisture sensor on the battery and inside the phone that will indicate if it has gotten wet enough to affect the sensor.
Other options - from when this topic came up awhile back:
The issue may be the magnetic strip underneath the keypad. I had an Alltel TouchPro and the keyboard would either duplicate a key I touched or nothing would happened. Back then, prior to my warranty replacement, I was told to hold all the keys down all at once or 20 seconds or so to temporarily fix the issue, which it did.
I am thinking when you get a refurbished device, those devices have never had the magnetic strip replaced, so when you use them, at some point the magnetic strip just dies.
You should be close to your upgrade as well.