Mine has been like that too. Sometimes the cable just falls out when I'm using the phone or it shows slow charging until I remove and plug in the cable again.My s8+ started to play up with charging...
Ned to push cable quite hard in for it to start charging....slightest touch to the cable and it comes out enough to stop charging...
And not always charges in fast charge...
Just got a new cable to see if it's the issue...if not, not sure what to do...