Electric dash, BCM problems in my '90 Riviera

I'm stuck. The electronic dash goes haywire on my Riviera. The temp gauge goes out, then the tachometer, then the cruise control and air conditioner, then the whole dash reads Error, the speedometer either reads 158 mph or 00, and the Service Engine and Electronic Malfunction light come on. I took the car to several Buick dealers; all but one said they couldn't help me. One dealer gave it a try. After having the car for a week, they said the power module was shorting and messing up the BCM, and GM doesn't make those anymore. I found used replacements online, and the dealer has had my car for another week. They installed the used power module and BCM, but this time everything went off after about 2 minutes. The service manager said they had no way of programming the BCM, and it may be the ECM that's the problem, but they don't know how to find that out.

Does anyone have any experience with this computer equipment from the 1990 model year, and know what to do?

This car is in great shape. Hate to give it up for something like this.

Thank you!