My 97 Chevy Silverado w/ auto transmission is having some shifting problems. Mostly when cold, it shifts hard going 1-2 and really hard 2-3. It is hard enough that it jerks you forward, like if you were dropping the clutch on a manual. When it gets warmed up after about 10mins of driving, it gets alot better. What could cause this? How concerned should I be? It has 152,000 miles on it and I just replaced the transmission fluid 2 weeks ago. It was doing this before I did the fluid change.