Things like beef heart aren't the healthiest thing to feed an axolotl as one of the staple foods. They can be fed bits of red meat occasionally (ie strips of liver/heart, snitzel, steak etc... as long as they all fat/tendons have been cut off and they're cut in wormlike or sliver strips). Strips of raw fish/shrimp can also be fed, but don't use feeder fish. Feeder fish would need to be quarantined for at least 30 days to ensure they're healthy, and a better idea would then be to breed from the feeder fish so you know you have a healthy food source for your axolotl.
How often do you waterchange and do you test your tankwater at all for ammonia or nitrites?
Last year as in 2007 or previous 2006?
Cut back feeding to once a day - evenings or early mornings, and try and get some alternative foods as suggested above; when it starts to cool down feed once every two days. Axolotls will try and consume whatever they're given - they're opportunistic eaters and will try and eat as much as they possibly can given the chance. Ours generally get 1-2 earthworms with a few slaters thrown in every two days at the moment; if they're lucky and I find enough live bloodworm they get some bloodworm too. Otherwise the live bloodworm is kept for our axolotl larvae.