When I lived in Mount Pleasant, I used Mount Pleasant Auto occasionally, and my experience was the same as yours. They do their best, but they aren't very efficient.
I now go to Westbard Citgo in Bethesda. (Yes, it's a gas station, but they do a huge repair business.) They are reasonably priced, honest and--more to your point--they have always finished by the time they promised. Once, when I had an old car with an oil leak, they diagnosed the problem and adviced *against* paying to fix it. I was prepared for the expense, but they persuaded me it would be cheaper just to keep adding oil, even though it would mean no profit for them. (I added my own oil.) Anyway, check them out. I think you'll be happy.
Westbard Citgo is located between Massachusetts Avenue and River Road, about a mile past the DC line. From your area go out Mass Ave into Bethesda, go under the overpass (you'll see it) and make the first right, onto Westbard. It's one very long block on the right side.
Westbard Citgo
5471 Westbard Ave
Bethesda, MD 20816
(301) 654-4140
Good luck!!