2000 Buick Century Electrical issue

Hey Guys, I was wondering if you could help me out here. I have a 2000 Buick Century 3.4 with a frustrating problem. When the car is running the dash lights (all of them) randomly come on
and off driving or sitting still and the needles on the dash will randomly go all the way up then all the way down. No codes stored in computer and communication with computer is good. I am using Matco pro scan plus to read PCM and BCM data. When I connect the scanner the car it does not act up at all. I checked all power and grounds I could find using a power probe and they seem good. Battery is new and alternator seems good. Readiness monitors are good but Oil life keeps resetting it self.
The fact that it will not act up when the scanner is plugged in and reading PCM leads me to believe it is the PCM is Bad.
If anyone has run into this or has an idea for me let me know please because I want to know I am right before I buy the PCM cause it is kind of expensive.
Thanks in advance, Chazg32
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Check all grounds and connections for loosness and corosion with a wrench and screwdriver. Just pressing on a loose connection with the power probe will cause this type of problem to look good.

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There were some common problem with some of the headlight switches from that era. Does yours have a pull out head light switch on the dash? If so, they go bad and one of the symptoms is that you lose the dash lights. I think the switch is around $40 at the dealer and installs pretty easily. Don't hold me to that price though - it's been a couple of years since I replaced one.
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Thanks Guys aprciate the advice I did rule out headlight switch though the back lighting works ok, I have done wiggle test under hood and dash. I think I am going to remove the cluster and check it out before anything else.
Thanks, chazg32
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Thank you to those who responded!
Sorry it took me so long to update but here it is.... I found 2 blown resistors on IPC (instrument pannel cluster) and so I replaced it (not once but twice) with junkyard cluster(s). Still same problems (the car must be warm to act up) the coolant temp gauge and fuel gauge go all the way down and back up again Veh. Service Soon (not check engeine light) comes on random check engine light is on once vehicle warms up and random security light. when problem is occuring I lose all communication with the veh. using my scan tool. used ohm meter and tested all wires to the cluster, used power probe to give good ground to common grounds and dash grounds to rule out faulty grounds.... and so on (I have mitchel wiring diagrams for the whole car that I have been using)
This car is my mother in laws and after way too much time now I have told her I think we need to try the computer, I will let you all know how I make out on this one!
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