The 3300 mAh Nimh should handle the approx 10 A that are being drawn on the start up . I use Nimh on all my Wrens and they work fine . There is the possibility that this particular battery is not delivering . Check the voltage of the pack under load . Also confirm that your autostart settings are in fact commanding the starter to at least 30k rpm . If the start was progressing on kero and hanging at 22k I would actually expect that the temps would start going high . Make sure that the engine is getting kerosene . No blocked filters , kinked lines and that the kerosene solenoid is plugged into the correct port if you're not using the propane solenoid ( I don't). Good luck !

Marc