Toyota Corolla- electrical problem?

1996 toyota corolla, 1.8, 160,000 miles
Pretty frustrated. Car had been running fine. About 1 1/2 years ago car died on highway with no prior symptoms. Small shop replaced condenser and it ran just a few miles and then started missing(more of a hiccup or hesitation, very noticeable) badly and then died a few miles later. Took to different shop and they said distributor was bad and replaced it. Car was running again but still missing some. Less frequent when it warmed up. Same shop tried a number of things:O2 sensor, vacuum lines, and more, but could not solve the problem. Took to Toyota dealership. They put on Toyota plugs and wires and another used computer they had. It improved but was still doing it some. They felt the problem was electrical and could probably find it but were not sure how long it would take. Had already spent several hundred dollars and did not want to
spend much more on a car not worth much more than $1000. That was in April, 2010. That summer the missing improved to almost nonexistent. With cooler weather it got worse and stayed about the same through the spring. The next summer(this past summer) it did not get better and has gotten worse this fall, missing/hesitating more frequently, having loss of power occasionally. Mpg has only dropped 1-2mpg. This winter it remained bad with more times not having full power; almost always when under a strain. I really can’t afford to spend any more money on diagnostics. Any thoughts? I am wondering what has been happening to cause it to get worse.