What are you using to control your sampling timing - are you relying on a hardware or software timer or the connection interval? At 5ms this is faster than the minimum connection interval. What is your connection interval and how many samples do you send per connection interval.
If your application is very sensitive to subtle changes in sampling rate, you need to control the sampling rate through a hardware timer and make the communication timing independent of it as there will be slight variations in packet length / retries etc.