This may be a result of spline interpolation between keyframes. Too many keyframes too close to eachother will result in some wacky curves between points. Select the light, open your Graph tool and select the Intensity channel. Here you can see a line graph of the value of the intesity over time. You may be able to fix this by spreading out you keyframes and making the values more subtle at each keyframe, or by changing your keyframes from spline to linear interpolation.