Random is great for the first spell, IMO, but once you have it on 3, you only have certain gaps which heal instantly. Example using OP's settings:
98~100, nothing
92~98, RANDOM exura
79~91, exura
72~78, RANDOM exura gran
54~71, exura gran
47~53, RANDOM exura vita
00~46, exura vita.
As you can see, there is a total range of 18% of health where you will heal randomly. Considering the fact that you should never drop below 46 (unless you're dead) without healing, this means that there is a 35% chance (18% / 52%, given: [100 - 98 = 2] [100 - 46 = 54] [54 - 2 = 52]) you are in a range that is randomized. Because, on average, randomized areas will take ~4 seconds to heal (but an be between 0 and 10ish), it will seem like it is healing slow around 1/3rd of the time.
MAAAAAAATH.