Need help. Have a 900 size Vario R22. It flys great but am having a problem....

Need help. Have a 900 size Vario R22. It flys great but am having a problem. Burning up tail rotor servos. On the third one now and at this point it wont last long. After a 7 minute flight the servo is hot enough you can blister your finger (if you could hold it on it long enough) . Shorten the flight time to 5 minutes not much difference, still hot. The cyclic servos are just fine. What we have done so far in trying to fix the problem. Linkage operates freely. Checked the voltage coming out of the FBL ( same as the cyclic ones) it is within specs. Servo is a HV up to 8.4v. Double checked the programing in FBL. it is set at 760 pulse and 500 frequency.( servo specs are 760 pulse and 560 freguency but Axon Fbl only goes to 500) Have run out of what to check or do next. I know I can't keep replacing servo at 2 or 3 flights,