I have a '99 Civic, automatic, and I was wondering why it stutters at a certain speed. It seems that my car does not like 48 mph. At 48 mph (especially if the air is running), the car will stutter. It stops as soon as I give it more gas or let up on the gas, but at 48 it seems to have a problem. Any thoughts? (The car also doesn't like to start when the gas is at 1/4 tank or below, it chugs one or two extra times.)

On a separate note, I allowed myself to be talked into having my transmission fluid flushed and changed today. Was this a good choice? (The fluid kinda looked like dirty water, but I have heard conflicting views on this one.) The mechanic recommended flushing the fuel system when I mentioned the issue I have detailed above, but I decided to wait on that.
Thank you for your consideration.

 

Likely at 48mph you're hitting resonance with off balance tires or axles.

I dont think the transmission flush would solve this problem, but hey new fluid could be good for the AT.

 

^I can vouch for that explanation. I had some tires with an uneven wear pattern due to balance issues. At exactly 31 MPH and a certain RPM range, the whole front end would shake. I got some fresh tires, balanced them properly, and the problem went away.

Transmission flush is a good idea. If the fluid was dark, it was probably time to get it done.

The fuel system isn't likely an issue. Just fuel pressure when the tank is low. Instead of turning the key immediately, turn it to the ignition position just before it would start, wait a couple of seconds, and then turn the key. See if that makes a difference. (This lets the fuel pump prime for a few seconds.)