I had this same problem a couple of months ago. My mechanic replaced the ignition module but now had the problem again. Car begins to jerk while driving at high speed. All electrical systems shut down-dash, all lights, everything. Also get a message BRAKE FAILURE-STOP SAFELY?? This is an extremely dangerous situation for myself and my 19 year old daughter! This car has been a nightmare with one problem after another. I really miss my 1994 940. Any help would be appreciated.

Your mechanic just threw a part at it without knowing what was wrong. You need to find someone with a Volvo scan tool that can show you the error codes involved. I would assume that the fault lies with the CCM under the dash, this module is prone to overheat and cause symptoms similar to yours, but it is expensive to fix it properly.
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Keeping it running is better than buying new

Thanks very much for your response. Just want to clarify you mean the CCM (climate control module) not the CEM. I have been doing a lot of research on this problem and it seems it is unfortunately very common. Also mentioned a lot is the ABS module. My mechanic is a former volvo mechanic and has been keeping this car running pretty well for the last 10 years. Of course he cannot get the car to repeat the problem. Also I'm not sure if the scanner is an official volvo scanner though, I thought it was. I guess I could get it to the dealer to just scan it if they do that.

Sorry, I meant the Centeral Electrical Module, the one that handles communication between all of the onboard computers. Not the climate control.

From Xemodex: CEM supplies “Wake-up” power to the BCM(brake control module) via fuse #15. When the CEM is defective, there won’t be any power coming to that fuse with the engine running.
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Keeping it running is better than buying new

That makes sense. What is strange is the problem happens so sporadically, every couple of months. Wouldn't the CEM just go or does it happen gradually over time? I should explain I do not drive the car more than 4,000-5,000 miles per year, very short commute to work. However, I drove it last week on a trip about 2.5 hours one way. The car started acting up the next day when my daughter drove it. Had to tow it home from our trip. Thanks so much for your help, you really know what you are talking about!

Two common reasons for failure: water gets into the cabin through the cowl and ruins the CEM circuit board. Or, the pitiful little heat sink in the CEM gets too warm and the circuits get overloaded.

Because you don't drive much, I would expect the latter.

A good scan tool will tell you which of the two is at fault.
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Keeping it running is better than buying new