So, there is really only one answer here. Especially lacking a graphics card that could be unwilling to play nice with certain power supply models as some of them have issues with each other.
If the system works fine with an old, lower capacity VP450P, then there is no way it should be having issues with a HCG850. There must be something wrong with the PSU itself unless you are failing to plug something in or have plugged something in wrong at either the PSU or motherboard ends.
If you've triple checked all connections including for loose pins in the ends of all cables (Sometimes a cable male or female pin/receiver may not get fully locked into position inside the connector and might not make contact with it's counterpart in the other half of the connector/socket relationship) and nothing is off, then it pretty much MUST be the power supply itself.