Trouble with crochet gauge?

I've only crocheted afghans, scarves and hats so far. I would now like to try crocheting tops and cardigans. The pattern I'm currently wanting to try says the gauge should be 27 sts = 8", 8 rows = 4" in dc. Also, the pattern says the width of the body before sleeves and edging should be 40". I understand what it all means and when I make a swatch of 27 sts/8 rows using the 5.0mm hook, it comes out right. Then I try making the 135 sts for the full row and it equals 40" wide like it should. But, by the time I get a few rows up the project gets slowly wider and wider. Even though each row still has the correct amount of sts. I'm doing my best to keep my tension the same throughout. I've looked closely and all the stitches look uniform. How do I make it all come together properly? It surely shouldn't be this complicated to do. I've tried out different sized hooks from 5.0mm (what the pattern suggests) to 3.5mm. I go about it in a relaxed manner that's comfortable for me and flows quickly. I just don't understand why the project gets progressively wider. The only thing I know to do is take out all the stitches and start over when it starts to go wrong. This is after already completing about 2hrs worth of work. :/ I sure hope someone can help me solve this. I would like the opportunity to maybe communicate through email to further help as needed on this project. Thanks!