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It's hard to pinpoint the issue just from a graph. A glimpse of stars in on the images suggest these are highly elongated so maybe the centroid is not being calculated correctly? Also, 1.5s guiding exposures will be more strongly affected by seeing, so trying to go a bit longer (2-3s) might help in pinpointing the issue. There could be a few factors contributing to those spikes. But certainly improving the accuracy of the PA is a starting point.
Recently I was getting sudden spikes with my Mach1 and it took me a while to realise it was my guide camera not the mount or anything mechanical - every now and then a random subtle noise pattern would appear in the guide camera 's exposure and sometimes this noise would overlap with the guide star distorting it and consequently causing phd2 to miscalculate the star's centroid. Lowering star mass tolerance change fixed that, at least until I will get a new guide camera. This experience taught me that a good low noise guide camera is essential in getting the best out of the mount.
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