Steven,
chances are that it is NOT crabgrass at all.
Grab grass is annual: meaning it germinates in the spring (you can't usually notice it until early summer), and dies at the first frost.
So, if you see that "crabgrass" in late April, then it is something else like Barnyardgrass, goosegrass, or clumping tall fescue. (don't confuse this one with Turf-type tall fescue which is good)
If you have the big boy companies out there putting down their treatments, and you still have that "crabgrass," chances are you have another problem.
The only way to get rid of barn and goose grasses are to dig- em' up.
Also, you need to realize that these grasses are only able to invade weaker turf areas. Strong thick turf will choke out non-desirables.
As far as some areas of the lawn getting thicker, and others not, you very well may have some new species of turf grass beginning to take over.
have you introduced new seed anytime in the last 5 years?
My advice short of scraping up the lawn and starting over: (cost assuming you are sitting on about a 5,000 sq ft lawn)
1) put down a nice winterizing fert now. 20-20-10 will do nicely. ($15)
2) put same stuff down again in mid-march. ($15)
3) slit seed entire lawn with nice blend of KY blue and Perenial Rye..also mid-march (seed $100, machine rental $50)
4) put down crabgrass pre-emergent in later april. you can buy a blend with weed control and fert in it too. ($20)
5) continue putting down weed-n-feed every 4 weeks through summer. ($45)
6) aerate and overseed lawn in september. (seed $100, machine rental $50)
7) finish year with 20-20-10 in late fall. ($15)
total cost: $310.
approx cost to scrape and start over with new sod: $3,500
also: cut grass at 2.75" and NO shorter (free)
water lawn 2x per week even if it rains. When you water, leave sprinkler in each area for 1 hour. (water is cheap)
If you do this, your lawn will show a marked improvement. It sounds like a lot of work, but really isn't.
Perform the above work for a couple years, and most of the crappy grass will be choked out.
hope this helps
AL