Had the same thing happen on mine recently. Reliable starts, hot, cold, warmed up, stone cold, sitting for 6 months, multiple stops to shop. Rr-rr-RRUMMM-RRUMM! 20000 miles on first DT300i, 6900 miles on the second. Then one day God says I ain't gonna ride that day. Posted a thread on this in Tech, "Troubleshooting no start on DT300i" I think. Dee dee says check the regulator but it sounds like an intermittent. I did check the voltages and they were normal: 12.3 no load, 11.3 key on and 13.6 idle. That range is pretty normal from my experience but may be out of range for some of the others. Start circuit works fine as cranking (starter) speed sounds normal and warning lights are normal for the ECU being energised. One rider discovered his plug wire came off. No, there is no CONVENIENT way to jump-start one of these!
One thing I did on my first DT was to lather up EVERY connector with silicone dielectric grease to delay corrosion. I have lathered up SOME connectors on this replacement machine.
There is a train of thought here on this forum that the ECU is really fussy about low voltage. That if your cranking voltage drops below 10.0 the ECU won't fire the plug and I agree with that thought. So much so, I was ready to replace the battery.