slamming into gear
 

I just bought a 97 civic a little over a week ago (auto transmission) with 129000 miles on it it runs great other than some hard shifting. When I test drove the car I didn't notice the problem, but after driving about 30 miles it started revving really high when it's shifting into 3rd and then it slams into gear it has done it a few times shifting into the other gears but mostly 3rd. There are no warning lights lit in the dash. The fluid level looks fine, it does look dirty though. It only seems to do this after it has been driven 20-30 miles. Would flushing the fluid help this problem or does it sound like something bigger? If flushing the fluid might help what is the best way to do it? I've read not to have the machine flush, I've also read to drain and fill 3 times but some people say only once cause the detergents will remove too much gunk at once and will hurt it, so to do it again after a few thousand miles. So how should I do it? Please Help!