To stop most weeds, including crabgrass, cut your lawn at 3 or more inches in height. This stops weed seeds from getting the light they need to germinate. Only water once a week, heavily about 1". This makes the grass grow deep roots and it will be able to withstand hotter temperatures. It also lets any weed seeds dry out and die before germinating. What is happening now is that your grass is probably going dormant when it gets hot and does not grow. This allows other weed seeds to germinate and grow. Frequent light watering is what you do when you WANT to grow seeds.
You can use corn gluten meal (CGM) as a pre-emergent organic herbicide. It is also an organic fertilizer. Apply 20# or so per 1000 square feet of lawn about when the forsythia blooms in spring.
If you stick to this program as well as fertilizing every couple of months with organic meals like alfalfa or soybean meal your grass will be very thick and not allow weeds to grow because they will not be able to compete with the grass.
Crabgrass is an annual weed, so it comes back from seed every year. I have found no organic way to kill it, and most chemical means do not work either, Especially not any ortho products for crabgrass from what people are saying. So stopping the seed from germinating is really the only viable solution.
For more information you can check out the lawn forum.

Lee

Timing of preemergent applications is critical and most need a second application a few weeks later. Be sure to read the label on whatever you are using and apply accordingly.

Seeds don't all germinate at the same time and some will remain dormant longer than others, some of them for years. The war against crabgrass can't be won in just one battle. And seeds from your neighbor's yard can blow in or be brought in by critters. Keep at it and you will get a good result but it's an ongoing problem in most yards.

Sandy