The other issue you may be facing is related to A feature of the esc called lipo cutoff, which is why I asked about the battery. Most modern esc’s come set up to work for lipo batteries which will put out 8.4V (4.2 per cell) and are usually designed do best to cut off at around 6.4V-7V. Your older pack may dip below this cutoff voltage quickly, causing the esc to stop. This is designed to protect lipo batteries that don’t like being run down too low. You should disable the cutoff voltage for your nicad battery and stop as soon as you notice a significant loss of speed.