As Jeff says, have a ball valve on the pump output to throttle the flow. It is sometimes a good idea to close the valve, start the pump, and slowly open until you have flow (not full flow). Then turn on the heating element, and see if you can maintain flow.
Another issue is that the false bottom could be bottoming out under pressure. What that means is that the elbow contacts the bottom of the tun, shutting off flow. Some designs have castellated ends so that the flow is not shut off, some guys cross drill to give more flow area. Some coolers have a small dome in the center, which makes it worse - don't ask me how I know this.
It is generally thought that slots are better than holes. On a pro level there are many examples of slots, but there are some that have holes.