what will be causin my car not to be able to over 20mph? on 2000 Honda Civic

its like it aint in gear but it is plus i have used redex on it but seems to ave made it worse

Asked by for the 2000 Honda Civic
Auto/manual trans? V-tec? What is 'redex'? How many miles on it? Does it start and idle normal?
I have the same exact problem. Automatic trans, starts and idles fine, shifts into gear and reverse and moves but wont continue to go if i drive it, it seems to go into neutral, but if I shut it off for 30sec and restart it it will move again????