This seems to be a common problem for all UNO/GRBL users. And the answer can be pages long…
There is an excellent discussion on possible issues here: https://www.shapeoko.com/forum/viewtopic.php?f=3&t=715
And I will try to summarize some of it:

1. Power – the USB communication on the Uno seem to be very susceptible to electrical and magnetic interference. Having swimming pool pumps, air cons and even fridges switch on has been reported as possible causes. How do we combat this?
   •  Some users achieved success by simply powering their electronics and spindles from different circuits
   • AC Line filters to your PC and electronics 
2. Localized interference – Many users reported that having the stepper motor outputs from their drivers pass close to the UNO/USB cable cause interference (To date our Y-stepper motor wires passed right under the UNO and re-routing the might be time well spend).
   • Using a ferrite core around the USB cable is also highly recommended.
   • Optically isolating the USB/Serial communication is suggested
   • Employing a Bluetooth connection to the UNO is suggested
3. PC/PC settings – Obviously you do not want your PC to go into sleep mode while streaming G-code, furthermore going into the USB hub settings under device manager you’ll be able to “untick” the “disable to safe power” setting under the power management tab.
   • Sometimes another PC just works – reason unknown… one user for example found that his GRBL froze when his Wi-Fi link disconnected. (So, try a PC with as little unneeded hardware/software as possible
4. “Overloading” the UNO – There are theoretical limits to how many “pulses” the uno can send in a given time frame. If you for example employ a rather high level of micro stepping on a threaded rod with a high feed rate, chances are you might be exceeding that limit.
5. There are users who believe GRBL 0.9J is prone to freezing.
6. There are users who believe the UNO should be “cleared” before re-flashing it with GRBL

For now, I think that’s the bulk of it