I have burned 3 throttle servos in less than a month.
Two in my buggy and one in my rallygame car.

I use ko propo radio system in the buggy and Sanwa in the car both with Hitec digital 5985 steering servo.

I burned one hitec 645 and one Jr 590 on the buggy and one futaba 9402 in the car.

I have my EPA set correctly so i don't know what has caused this.
Could it be that i use digital and analog servos together?
I have burned all three at the moment i press the brakes.

The Jr didn't last even a tank.
The 645 was there for 2 Gallons...same goes for the futaba on my other car.
Some say vibrations...but i used the rubber grommets.

Having a mix of analog and digital servos does not matter. My only suggestions is to check very carefully your EPA, even drop it down a few % just to be on the safe side. Also as said before vibration is a big killer. You mentioned that you use the grommets. This is a good thing. I will suggest that you use high end servo's. I have had a few servo's go bad in my buggies. One I pulled apart and looked at the inside of it. I found one of the circuit board tracks was cracked. This is from vibration. I have found that the lower spec servo's are less resistant to vibration.

I haven't opened up all three but the one i did(hitec), had a black board which i suppose was from burning up.

I would be surprised if the something on the board burning up was due to vibration. I would reduce your EPA and get a quality servo. Also check for binding which may be fighting the servo.

Could the crystals be causing this?
I changed to Central crystals from HK to get away from known frequencies.Although i haven't notice anything on my previous buggy using the same crystals.

I would think that the crystals would only cause this if the new crystals caused the tuning to be slightly off frequency which may lead to the servos chattering. I think you would probably notice the poor range though. You might want to do a range check for your own peace of mind.

They don't seem to make any noises when on "idle".

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.