'98 Bonneville trouble

I'm having a problem with my '98 Bonneville. Less than a year ago, I replaced my engine (long story), and since then the Check Engine light has been on. It goes off for a day when I bring it in for checkups, but it always goes back on. The mechanics always tell me there's nothing wrong with the engine, so I've just been ignoring it.

However, this afternoon, I went to start my car, and I successfully got it in reverse and began to back out of my driveway. Then the Check Gages light went on, the dash started beeping, and the steering froze up. (Usually the gages light only goes on if I'm low on gas, but I just filled it up a couple days ago.) I turned it off, hoping it was just stalling or something, but then when I tried to turn it back on it wouldn't start, it just revved without turning on. It turns on halfway, which only turns on the radio and lights, but it won't go the rest of the way. When it's on halfway, the Check Gages light and the Check Engine light are both on. I'm bringing it in tomorrow to get it checked, but I wanted to see if anyone had any clue why it's doing this.