97 XJ6 VDP loses power on highway....HELP!

I have a 1997 VDP I bought a year ago with only 39K miles on it...literally a "little-old-lady-only-drives-it-on-Sundays" car. I love the car. I now have 50K miles on it.

There is only one problem: twice now it has lost power on the highway. While at highway speed, suddenly the gas pedal only gives me about 20% of what it should...my speed starts to drop, and I have to put on the hazards and pull off the highway...very dangerous. The engine gets gas, just not as much as it should, as if the fuel injection has given out.

The first time it happened, I got off the highway and stopped the car. On restart, the problem continued. Only after I let the car rest about 15 minutes did it start up fine again and act like nothing happened.

Second time, I got onto the shoulder of the highway, turned the car off, restarted, and it was fine immediately.

First time was about two months after purchase with about 40K on the car, second time was last week with 50K on the car.

The problem is that this occurs so intermittently....about a year apart. I get no lightup signals on the dash that there is a problem, and no Check Engine light which would give my mechanic a code. He is frustrated: he wants to solve it as much as I do, but says the possible causes are too myriad...we could spend a lot of money looking for the problem and unnecessarily replacing stuff.

Does anybody have experience with this phenomenon that could provide me some hints? I don't want this to happen again...it's too dangerous.

Possible other clues: I do get electronic eccentricities on the car...for example, the gas tank will read "EMPTY" even when it's full, or occasionally the engine will read all the way to the far left "COLD" even when it's been running a while. Given a few minutes of driving, tho, these mis-readings generally pop back into accuracy.

Any ideas? Thanks much in advance for your help