I have ruled out any tank problems. I have had the same problem with different tanks and the current tank is new and has been tested for leaks, proper clunker clearance, kinked tubing, etc.

The engine starts easily. Idles good, doesn't hesitate when going to full throttle quickly from idle. But when you put the throttle approx. half open, within 5 to 30 seconds the rpms start going crazy. The rpms will drop out as if the throttle was suddenly closed and then the rpms bounce back up to where they were. It surges back and forth like this every couple seconds until it just dies.