Warning
When in the nether, avoid using a jetpack and slime boots together to avoid server crashes


I'm not sure which factors are dominant, but the combination of mekanism jetpack and slime boots in the nether seems to cause instability in the server. I suspect the rapid speed changes of the slime boots and jetpack combine to cause positional errors, as using either on their own seems more stable. I haven't tested this with other kinds of jetpack (e.g. IC2 jetpack), but I suspect they would cause similar issues.

Switching to an alternate method of transportation (draconic armour, which provides creative-style flight) has basically solved the issue for me, which seems to confirm the suspicions surrounding the boots + jetpack. Alternative (cheaper) creative flight is provided by Extra Utilities 2 with the Angel ring.

If you've not used Extra Utilities in MC 1.10+ before, look up a recent (minecraft 1.10 +) tutorial on the Extra Utilities mod for an overview of the overhaul carried out between MC 1.7.10 and 1.10.2, in particular the "Grid Power" system, as the mod works somewhat differently as of MC 1.10 and requires Grid Power in order for the Angel Ring to work.