I don't know alot about Mechanical Speed Controls but I would
check to make sure that the servo is not travelling too far.
If the contact is lost you would loose at full throttle.
Hold the car in the air and check the MSC servo operation.
Also make sure the contact surface is clean. (use some contact cleaner) You may have to realign the servo horn.
When you get the funds upgrade to a Electronic Speed Control.
I would recommend the Novak XRS......inexpensive, small size, good quality, reversible, good down to 15 turn motor on 6 cells, you will also get extended run-time with an ESC.
Good luck with it and I hope this helps.