One item that I noticed that improved performance was a new transmission mount. Probably seems off base of what you are looking for but I noticed a big difference, perhaps not so much in the mileage category but maintaing highway speed without constantly feathering the throttle was the big difference.
A post that I found interesting concerned the brushes in the alternater.OP claimed that insufficient voltage was being produced and his/her brick was running rich. Good luck!

Knock sensor and related connections could be an issue as well! (rough idle, low power, bad economy)
Please see my earlier post how to pinpoint the problem.
Only in very rare cases the ECU and even a sensor fails (but they do fail), it is far more likely a connection problem. In my case I had all kind of connection problems, you can easily test it with a meter. You might get away with a cheaper meter, but something like the general fluke 87 or the more expensive automotive 88 could be it; depending on how much you want to get into it and what works better for you. I bought 3 cheapies before, so I wasn't that smart to begin with. But if you are (unlikely to me) not keen of electrons, it might be better to get somebody look after the car.

All your mechanic needs to diagnose a bad ECT, or wiring issue to the ECU ,is a good digital multi meter available for about $90-150 for a quality pro model.
If that is over his head, or tool budget, I would suggest a new mechanic.
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Gary Gilliam Sumerduck VA, '94 940 na Regina 160k '86 240 190k

So no error codes? Anyways, pull the vacuum hose line on the fuel pressure regulator and smell for fuel. If you do, it's a bad regulator, and can be a cause of all three stated symptoms.

-- Kane
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While I would never deliberately mislead anyone, take into consideration that any information and advice provided was at no cost to you.