[V8] Auto Tranny Question

[toml99 at todomundo.com]

Well I finally got to test drive the "parts" car that I bought from  
Karl back in October.  It's got 145K, and after getting it running 
(long fun story- later), I drained about 1.5-2 quarts of nasty out of  
the tranny.  I threw 4 quarts of Mobil 1 in and my dipstick said ok.   
It shifts fine, but on the highway in sport mode, it slips when  
kickdown.   I can start out in sport mode from a standstill, and it  
doesn't slip.  Question......Could a plugged filter/bad fluid cause  
slipping?....I know it can cause all kinds of bad shifting issues,  
but it shifts fine.  I know very little about Auto tranny's.  I don't  
really want to spent $150 doing the filter/flush, but would like to  
know if the tranny is ok.  Sincerely, Tom