Well first of all, slamming electronics devices against a table is never a good idea...regardless of how therapeutic it feels :>).
The iPhone 6 is well known for suffering from "searching" issues and ironically, it is believed to be caused by flexing of the logic board, causing microscopic cracks in some frail copper traces. The flexiness of the iPhone 6 and iPhone 6Plus (Touch Disease) make this more problematic in those devices. So smacking it on the table is only making this problem worse.
IMHO, this is an IC issue. The good news is that the repair community is confident at this point that a "permanent" fix is possible by doing the long jumper repair(U1/V1) (see No Service issue on iPhone 6 ). You'll need to seek out a reputable micro-solderer for this and most of them work on a no fix/no fee basis. Or you could upgrade your phone...but your phone still has value and someone should benefit from this device so try to avoid the landfill.