Safari sometimes closes new tabs before the new web page opens

Hi Y'all.

I have a bit of a strange issue here.

For a few months now i have had new tabs disappearing in Safari.

When i use command T to open a new tab it appears no problem. But sometimes after i type the new web address into the browser bar once i hit return the tab just disappears rather than opening the new address. If i then go to the edit drop down and reopen last closed tab it comes back but with the same web address that it had before i typed the new one in.

If i then try to repeat the process it sometimes disappears a second time but usually on the third attempt it will work and i can access the new web address.

This does not always happen, often i can open a new tab and then add the web address and it goes there fine.

Notes: i may add, I am connecting my macbook via wifi to a personal hotspot on my phone which is using 3G. It's not the best connection and sometimes a new page will become stuck and won't load up immediately. Sometimes this is helped by trying to refresh another tab or even try to open a web page up on my phone and then close it again.

Another issue i have had with Safari for several years now is that when i use Facebook messenger on the macbook the text in the message boxes at the bottom of the screen is all patchy, sometimes it's even slightly difficult to make out the letters like they are over exposed or have too much contrast yet my contrast is down and there is no such issue when using Firefox.

Macbook pro 13 inch late 2011

16gb ram

El Capitan 10.11.6

Safari 11.1.2