[fredjacksonsan]

What temperature thermostat did you put in? If it's too low, it could cause the issues you're having. What temp is the gauge showing?

If it's not overheating, you do NOT have a water pump issue.

edit: plugged heater core could do it, just disconnect the hoses from it, and put your garden hose against one of the inlets....make sure it'll flow both ways.