I have a 2000 grand prix gtp and one day I went to the store turned my car off and bought wat I needed came bak turned the car on applied the brake shifted into reverse and nothing tried all other gears and nothing it goes into like neutral stage they told me it was my tranny filter so I changed the filter and fluids brand new started my car and shifted into reverse and went in awesome so I took it out for a spin all gears went in so smooth even better then before the next day woke up to go to work turned my car on to warm it up waited lik 20min shifted into gear and nothing same thing again no gears wat so ever would go in I turned car off for lik 1min turned it bak on and shifted good I drove for about 15 feet then it went neutral stage again wat can it be?please help me!!!!!