Inconsistent Gas Levels

If you prime (add sugar) per bottle I would advise you change and batch prime as its easier and more consistent, method detailed below.

All I do to ensure a consistent carbonation level is use the calculator found here to determine how much sugar to add (corn sugar is dirt cheap) and then do as follows:

1. Weigh out the required amount of sugar into a pan
2. Add enough water to just dissolve the sugar and boil for a couple of minutes.
3. Cool sugar solution to room temperature and add to a sanitised bucket
4. Syphon my fermented beer on top of the sugar solution and gently stir to ensure good distribution of sugar
5. Bottle using whatever method you like and leave for 2 weeks at room temp
6. Enjoy!

One thing to watch out for is to ensure you beer has finished fermenting before you bottle. A typical rule of thumb is to wait for 3 consecutive hydrometer readings taken at least a day apart.

Hopefully if you've followed a good sanitisation procedure your inconsistent carbonation levels are NOT due to infection. This can sometimes show as a brown ring around the neck of the bottle and an off smell or flavour.

p.s. I found that the carbonation drops are expensive, terribly inconsistent and had other 'stuff' in so this may actually be the root cause of your problem.

Usually such odd gushers are associated with infection by e.g. wild yeast which have higher attenuation. You may want to do a side-by-side taste of normal vs gusher bottle.