It‘s a bit difficult to tell with the stacked images. Stretched single frames from dense starfields will be easier. But I do not see even distortions in the field, which points to miscollimation instead of tracking/flex issue. In the last image the stars are far larger than in the first two - this can be focus or seeing, masking the issue. If it really is intermittent, it could be a stressed mirror or corrector, due to temperature. To rule out tracking/flex take some very short and some longer exposures and see if the elongation gets worse with more exposures time.
If your parameters for guiding are correct (FL, pixel size), this is very good - what mount are you using?