1992 4 Cylinder cooling Gremlins!

My 1992 Voyager which I just acquired was overheating. After reading thru this forum, I changed the temp sensor and thermostat and hey presto! it does not overheat and the fans work good. Thank you all for that. Now when I drive it will redline the temp guage for about 5 secs. The warning chime comes on, and the heat starts to blow cold. Then it just goes back to normal temp again and the heat starts to blow hot. The last time I drove for about 30 miles and it did this twice.
I thought my heater core might have an airlock, so I worked open the hose and burped it and sure enough air came out.Air blows a bit hotter. But the problem still persists.
What's wrong? Is my water pump acting up ? I will be greatful for all answers. Thanks.:banghead::banghead: