2006 Uplander shifting problem

Hi our uplander sat for two weeks and now it does the bang into gear from a dead stop ( it has done that every once in a while from day one from the show room floor) but now it is very lazy between 1st and 2nd gear shifts. it has 135,000ish miles and never had the filter changed (yeah I know). Do you think changing the screen could be the answer? At $8.50 a quart I really don't want to waste it if its time for a rebuild.

Bill