SMS Delay (Samsung Galaxy S5 KLTE - SMG900F)

I currently have a Samsung Galaxy S5 (SMG900F). The phone is rooted and I'm using a Team UB 5.0.2 Custom ROM.

I've noticed over the past couple of weeks that I've began to experience a considerable delay when receiving text (SMS) messages.

If I long-press on the message and check the timestamps it occasionally shows up as "Sent: 20:41, Received: 03:20." An important aside would be to mention that this only appears to happen at night. If I send myself some sample texts from another phone; they are received almost instantly.

Has anyone else experienced similar issues and found a viable solution?

I'd also like to know if this is related to the ROM/Build, SIM Card, or whether it's purely down to the service provider (Virgin Media in this case).