I ran through a similar experience with an 88 volvo. You would drive around and the whole car would just die. The ultimate solution was in both the fuel relay and what volvo calls the "radio suppression relay". The fuel relay obvious what it does, but the radio suppression relay is actually a relay in the engine compartment controlling the fuel injectors. So that the radio doesn't pick up the rf of the injectors firing constantly, they run a lead to this relay from the key, and relay on the power to the injectors.
Both of these problems were pinned down to the little printed circuit board in the relay developing microcracks in the soldering. Solution is to reheat the existing solder to make if flow and generally the relay is 100% again.
Maybe want to give it a try. It is the true 2 cent solution. Look at volvo's www.brickboard.com
you find some interesting information in the faq's that could apply to you.