2003 Mazda MPV shaking when stopped, braking

I have a 2003 mazda mpv & the shaking when stopped is terrible! My mechanic has been unable to fix it (he's always cared for our cars). The check engine light use to come on when the fuel cap wasn't tightened, then started coming on every time I filled up, now it stays on. The shaking / rattling is the real problem though, I get nervous that the car is going to "die" on me and a replacement just isn't possible at this time.

Thank you for any assistance you can provide,
mkeller127@aol.com