2006 Honda Accord V6 slipping at red lights and stop signs, what's the problem?

Occasionally when I drive stop at a stop sign or light, or behind a car at one, the engine will rev at the press of the gas pedal but the car will not accelerate. Occasionally it will catch after a second if I press the gas slowly, but often I have to put the car into Park and then back into Drive in order to accelerate again. If the light is long enough, I inch forward by letting up the break to check if it will slip. When the car doesn't inch forward, I know I have a problem, and proceed to put the car into Park and then drive. The check engine light is not on and the local transmission shop mechanic told me he can't diagnose the problem if the light is not on. I'm not sure if the transmission is the problem so I want to have an idea before going to another mechanic.

New breaks

Check transmission fluid if low, and if is add fluid to fill to fill line, read in owners manual to see how to do this

Sounds like it could be a problem with your torque converter. If the transmission fluid level is low or if the fluid is burned, that can cause it to slip from a stop like you describe.

If the fluid level is full and doesn't appear to be burned or if you can't easily check the level or condition of the fluid, I'd definitely try taking it to a different shop and make another attempt to get it diagnosed before the problem gets worse and the transmission is ruined. Unfortunately not every transmission problem throws a code. Many don't, at least not until they're really, really bad. Some shops are better than others at diagnosing transmission problems by symptoms if a computer code isn't present. It would be worth a try to find one, it could save you the cost of a new transmission.