Slow thaw is great but how are you heating them up? Rodents should be 95-100 degrees when you are feeding. It sounds to me like the rat was still cold internally. More info. I used to feed F/T and the rodents need considerable time under a 140 degree blow dryer before I would feed them to my snakes. I get them to about 105-110 and then head towards the tub to feed. If you don't heat them properly this way their heads and guts can stay very cool still. 50-70 easily according to my IR temp gun. That's why I heat them with a blow dryer before feeding. It takes quite awhile to get a whole rat to the right temperature.
Regards,
B