I just responded to your email, but I will answer here as well.
I possible, try removing the offending frame for your light frames before stacking.
However, to "rescue" stacked
datasets like these, the Repair module is what you are looking for.
Repair: Star Rounding and Repair
^ The Repair module's "Warp" algorithm uses the original pixels from the image to reverse-warp stars back into shape.
The Repair module attempts to detect and automatically repair stars that have been affected by optical or guiding aberrations.
Repair is useful to correct the appearance of stars which have been adversely affected by guiding errors, incorrect polar alignment, coma, collimation issues or mirror defects such as astigmatism.
^ The Repair module's "Redistribute" algorithm uses the original pixels from the image and recalculates their appearance and position as if they originated from a point light source.
The Repair module allows for the correction of more complex aberrations than the much less sophisticated 'offset filter & darken layer' method, whilst retaining the star's exact appearance and color.
The repair module comes with two different algorithms. The 'Warp' algorithm uses all pixels that make up a star and warps them into a circular shape. This algorithm is very effective on stars that are oval or otherwise have a convex shape. The 'Redistribution' algoirthm uses all pixels that make up a star and redistributes them in such a way that the original star is reconstructed. This algorithm is very effective on stars that are concave and can not be repaired using the 'Warp' algorithm.