I own a 2011 VW Touareg, and I recently purchased a Samsung Galaxy S4 i9505. It was working perfectly until I turned on Bluetooth and paired it with the Touareg. The Bluetooth pairing itself also worked perfectly, but it caused the 4G data signal to drop out. Took me a couple of days to register that the dropout was always occurring when I was in the car, so I switched off Bluetooth and haven't had a 4G problem since.
I asked my phone carrier to replace my SIM card, but that didn't help.
I'm thinking that it may actually be a faulty phone, but all Samsung could suggest was to reset the phone to factory defaults, which doesn't thrill me given all the time I have already spent personalising it!
So, I stumbled across this forum, and got me wondering whether your "rSAP for Android" app may help me at all?