What's wrong with my G20?

[Johnston, James W]

Hey Guys, I need your expertice on a situation:
Please read this because I need to get this thing running smothly again
ASAP..and the warranty on the used engine I bought expires this month, so I
greatly appreciate your help.

Scenario:

-Temp Gauge in car is acting strange, going from warm (normal operating
heat) to cool rapidly (in about 2 seconds), occuring mainly when I punch the
accelerator.  Temp gauge has not gone over the mid range point.

-Last night, I saw steam coming from coolant boiling off the exhaust
manifold blowing from under the hood...quite a bit.  I killed the engine
right there.  TEMP GAUGE DID NOT REGISTER THE OVERHEAT.

-I checked my 'heat tab' they placed on my engine when I purchased and
swapped it and it is still good, so I did not burn it too badly...

I need to know whats up so I can possibly get this done for free.  Is it the
engine or a component?  Have I given enough info?  Thanks again for your
responses.

-jw 96G20