Volvo acting strange!

I have a 2004.5 Automatic Volvo S40 T5. It has approximately 60,000 miles and I just recently had the oil changed. For the past two weeks or so I have noticed a check engine light. When I read the code for the check engine light there are no codes to be read. The light would actually go off when I had more than a half of tank of gas, if it goes a little below a half tank the light will begin to come on again. Lately I seem to have a hard time cranking it when there is low fuel (lets say a quarter tank) normally my car is quick to crank, but it seemed as though it was slow and struggling. Normally when I try cranking it again it cranks right up with no problem. I thought perhaps the battery was bad (it was definitely corroded) so I replaced the battery. I'm still having issues though. And last night I was driving and coasting down a small hill and the engine shut off and all of the lights came on (airbag, check engine, oil, etc).I had to put the car in park and re-crank it, it cranked immediately. It runs like a charm, so I'm very apprehensive about what is going on here.

Does this sound like symptoms of getting bad gasoline? I read on a forum where sometimes a car will be hard to crank after sitting overnight because the water from the bad gas settles to the bottom of the tank, thus making the car struggle to crank. Also, the fact the engine light goes off when there is more gas in the tank is very strange. I'm not under warranty because I bought the car used (i've had it for 3 years), so please help me solve this problem!