As you probably know, Merc changed to a "Cool Fuel" system in 96 on the EFI motors. This was because the mechanical fuel pump that feeds the VST had a tendency to vapor lock. This sounds like what is happening to your engine. You can switch your system over with some new lines to and from the fuel rail, a fuel cooler, and a new electric fuel pump. Another option would be a low pressure electric fuel primer pump before the mechanical pump. When the system seems to lock up, switch on the primer pump and reprime the system. Or you could plumb in the electric pump to feed the VST and eliminate the mechanical pump from the system completely-just make sure you add a relay and some type of engine-off shut off so the pump won't run if the engine shuts down from a mishap.