i'm not sure about all the sevos, but i know the Hitec 6 range isn't the best for 1/8th buggies. they can't handle all the on off throttle action. the coreless motors are better. the 9 series is what i use with much better results. i went through a few 6 series before i had this pointed out to me. they're more expensive, but well worth it. i run both digital & analogue on one of my buggies with no problems.

as Grant said i'd check your EPA, a few % can make all the difference, i've learnt that one the hard way too. check that the servo isn't rubbing on the body & that the servo bracket's aren't too thick & rubbing on the case of the servo (i've seen this on the hitec servos when the thick red servo horn is used). another point to check is that the brake linkages are running freely & aren't hooking up on something else in the buggy like the splash guard.

as for vibration, it is a killer & if you use the grommets make sure you don't do the screws up too tight, if you do you can still get vibrations.

hope this helps.