redlines4us

I have a '93 Civic DX with automatic transmission. The top radiator hose was looking a little soft, so I decided to replace it. While I was in the mood I also decided to replace the thermostat, distributor "O" ring, O2 Sensor and changed the transmission fluid. The car was running just fine until I did all this except for the fact that it was pulling a 43 code (O2 Sensor). The new O2 sensor fixed the check engine light problem but now when I rev the engine or pull out from a dead stop (with the engine hot or cold), the temperature guage falls all the way below cold. When I let off the throttle or I'm at cruising speed, it goes to normal. It never goes higher than normal. I just opened the hood after driving it for about 1/2 hour and the coolant in the overflow reservoir looks to be boiling or bubbling furiuosly (guage never showed above normal). Every thing including the guage worked perfectly before today and the car did not overheat before today. Anyone ever seen anything like this before. I'd appreciate some suggestions. I can't drive it like this!
Thanks in advance! Bill