Yes there needs to be a capacitor to ignite the bulb, or "strike" it as the case may be, but that seems to not be the problem here. Electronic ballasts are capable of firing a Radium, however, they significantly under drive the bulb which leads to an early bulb demise due to the splattering on the inside of the envelope.
A probe start ballast will not fire a Radium, or any other "HQI" bulb for that matter, in fact IIRC there's no such thing as a 150w probe start ballast, the prob in that watt range is a 175w ballast (M57). The shutting down of the ballast could be heat related. For example, if you have a very old bulb driven by a magnetic ballast, the voltage to keep the bulb ignited exceeds the normal operating voltage, that creates strain on the ballast causing it to overheat and shut down. Ever see a HID street light that goes off and on constantly? That because public works needs to change the bulb, if they don't eventually that leads to ballast failure.
Just thinking out loud, the verbage on the unit its self is rather confusing as to WTH is in there.