temp sensor frustrations

hello from texas! i just joined this forum and i need some help. i have a '99 grand am se, 6 cyl and a couple of days ago the check engine lite came and the temp. gauge started acting wacky, (like just dropping down, then shooting up and so on. the car hasnt heated up, but i noticed that when the temp. gauge would act wacky, then u could "feel" the car shift. so i took the car to autozone to get checked on their diagnostic tester, and it showed that i needed a temp. sensor. so i got it. and had it installed. i then disconnected the battery to reset or erase the "check engine" lite. it did. and drove the car to which it started doing the same thing. went to advanced auto to get it checked there, and same thing came up..."temp sensor" i bought it, (thats 2 now) and had it installed. drove it and for about 6 blocks it ran smooth and the temp gauge started acting normal again. then all of a sudden..."plop" down came the gauge again and check engine lite came on, feeling the car shift as well. so what gives? if the diagnostic thing says that...and i already bought 2, what can it be? i realize that those dinky testers arent as accurate as a super size dealer computer, but does anyone have any ideas or suggestions??? i'd really appreciate it. thank baggyshorts :banghead:

This engine has 2 coolant temp sensors. One sensor is form the PCM and is mounted right near the t-stat. The other temp sensor goes to the Temp guage and it is mounted into the rear head on the right side of the engine.

The PCM snesor is a 2 wire sensor and the guage sensor is a one wire sensor.