97 960 electrical system problems

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I have a 97 960 wagon. Car is in excellent condition, but I have something strange that the volvo specialist hasn't been able to figure out the 2 previous trips in.
On occasion, usually when flashing lights onto high beam at night, the entire electrical system goes haywire...tach jumps up and down, radio switches off then comes back on, and the transmission light on the dash lights as the car lurches. After 5 or 10 seconds this goes away and everything is normal again.
Also, the transmission has taken on the habit of "searching" for gears usually after I have driven 20+ minutes.
These problems seem related...it seems like a power "spike" goes through the car and affects all electronics under load. The volvo shop replaced the battery last time and said they found a short in the old battery. That seemed to take care of the problem for a month or so, and now it is back.
Please advise if anyone has any ideas...
Tks

[LaRy]

As I mentioned several times before, when odd things happen in a car at the same time, it is most often a ground fault. The ground (minus) has bad connection somewhere. I would check and clean the groundbars in the car. You will find two big groundbars under the hood at both sides close to the front. Check all flat AMP connections for oxidation and loosen also the grounbar itself and clean it by sandpaper or steel brush so you really get a good connection to the body of the car. Often the flat connectors are also getting "tired", giving bad connection. In the cabin you will find a major groundbar behind the panel to the right in left footwell. Check especially that groundbar that all flat AMP connectors are tight and have a good connection. My personal guess is that you will find the problem there.
If the ground is bad for some power consuming unit like headlight, the ground will always try to find a way to ground, unluckely through some other components causing odd functions.