What causes a 1999 Vanden Plaz to have a serious mechanical issue?

I have a 1999 Vanden Plaz that might have a serious mechanical issue. Both the "restricted performance" and "check engine" light is on. This has happened twice before. The first time, after sitting for a week while on vacation, I came back and it would not start. I towed it to a mechanic who stated that the spark plugs were soaked with oil due to a bad gasket with a broken seal. He replaced spark plugs, replaced the gasket, and a few miles down the road, the lights popped on again and the vehicle would vibrate when at a stop - it was doing this prior when first noticed the "restricted performance" light. I did not take it back to the mechanic. Thirty days later the vehicle would not start. I towed it to another machanic (having moved to another state). This time the mechanic was found at a Jag dealership. They got the vehicle started (changed plugs again). After leaving the shop - light comes on again. I turned around - the mechanic kept my Jag for the day but could find out why the light is on. Frustrated, I picked up my Jag and drove home. I drove nicely, but still vibrated at stops. Now 4 months later, I had my Jag garaged for about 3 weeks. After three weeks, it would not start again. The spark plugs were pulled (they looked bad) and cleaned - my Jag then started right up. No one knows whats causing this issue nor the solution. Can you help? Should I just sell it? I still owe $4,500. I can't afford to spend $800 every 2 to 3 months....