@Corkman This is very strange indeed. Have you had a chance to check your SIM in a different phone by any chance? Also, do you notice this in all areas or just at home?
@Corkman Let us know how you get when you get a chance to try the SIM in another phone. Also, how long have you had your phone?
Can you goto settings --> Netowrk and Internet ->Mobile Network -->Preferred network type
Set it to 4g/3g/2g