A friends SVX had a simiar issue. This was so simple:

- Restore 6 cylinder engine oil treatment to improve compression (change oil in 1500 miles, use restore with 10W-30, then go to 10W-40 after for a smooth transition)

- Restore Fuel System cleaner and octane booster (use with octane closest to 91, not garbage quality gas)

Lots of rough running until gas was out of tank. Next Citgo 93 octane fill up: smooth as a new Lexus.

Now every oil change, I use the Restore Fuel System cleaner because of her results. Can't hurt.

Once, I even tried this Oxygen Sensor cleaner when I got my SVX, just for preventitive maintainance. Didn't do anything, but still makes me feel better knowing my oxygen sensors are maybe a little bit cleaner. Be careful not to wait to put it in the gas tank. I did, and in about a minute the bottle was noticably lighter than when I put it down. After I put it in, it took only a few seconds before the bottle was dry as a whistle.