We just put in a new pressure tank [has a bladder] & a new hot water heater, [had good water pressure before], now we start with pressure, but after a minute or two it goes down to 0 , then back up, then back to 0, etc...

Make sure your pressure in the new tank (as measured at the fitting at the top of the tank) is set two pounds below your cut in pressure on the switch. Also, at what pressures is your pump cutting on/off?