Just an idea! Since u replaced the starter motor, check that the wiring isn't shorting out behind the block. I had that happen to me once where the wires came lose and melted the insulation by touching the hot engine block and it would ground itself and shortcircuit the engine to a stop. It started cutting out while moving at first, till it would stop and then it would not start again. Be very careful with wires near the started as the amps there are high enough to harm you. Disconnect the battery and check the wiring that runs behind the engine and under the inlet manifold to the starter!