Hi Kate,
It is strange that the two brightest stars gave the ‘noisiest’ spectra. And also strange that you got a good spectrum of beta Peg in between the two noisy ones so presumably the problem wasn’t due to some step change in the set-up. All but beta Peg are hot B-type stars so again, no correlation there either. Maybe there was patchy cloud on the night which reduced the light from the two noisy stars? The fact that you obtained good quality spectra from two of the stars, event the faintest, suggests that fundamentally all is well with your set-up
Cheers
Hugh