Have you tried shutting the scanner off and letting it sit for a while. Then turn it back on and see if reception is improved. If it is, let it run for a while to see if you lose reception again.
If you do lose reception, it's a hardware problem. Not a SDS, but I had a 536 that would go deaf at random. I finally figured out that it was a thermal problem and the hardware would fail once the scanner had been running for a while.
I sent it to Uniden and the replaced the bad component. It's been a few years so I don't remember exactly what went bad.