The jump at 250K looks like bad contacts to me that fixed themselves during cooldown. Rewiring the sample and remeasuring should get rid of it. The field dependence is indeed a bit weird. One would expect the critical temperature to reduce with increasing magnetic field (magnetic field weakens superconductivity), but here you don't see that. It's expected that the superconductivity is very strong for such a high critical temperature and the fields applied are not strong enough, but with better measurements you should still see a small reduction in the critical temperature.