Did this just start happening when you switched to sprinter? Make sure you have the right thermistor selected in your firmware.
Also I'm pretty sure the skeinforge setting for heating rate in only affects how long it will orbit, waiting for the temp to increase. It doesn't actually control the amount of time it takes to heat up. The idea is if you time how long the heating takes and put the number in there to get an accurate wait time for the temperature change.