2001 Buick Century cutting out

Tires and Wheels problem
2001 Buick Century 6 cyl Front Wheel Drive 186000 miles

I have a 2001 BUick Century that has 196,000 miles on it. I have always fuled at the same place till a few wks ago. Now it seems that my car is cutting out on me. Sometimes it will go 90+ and other times it will not go over 45mph AHHHHH! I changed the fuel filter seemed to do better for about 30 min and now back to cutting out. I did put some fuel treatment in the other day. I really did nt notice a difference. What else could it be Thank You