'99 Intercpt. Lights Blinking Engine Misfiring All At Once

Help..... I'd appreciate any input on this one.

2 weeks ago the 'check engine' light came on..... before I could get it in the shop, I had this weird thing happen.

I was driving in the rain so I had the lights on when, out of the blue, the dash lights and headlights started blinking.... when I tried to pullover, the engine was misfiring so bad I thought it was going to die.....

I took it to the shop where they replaced the gas cap which took care of the 'check engine' light and the dash and headlights no longer blink, but I'm afraid to go very far in the car because of the engine misfiring so much....

Any help is much appreciated!

Thanks