1997 Buick century loses power after driving for a while

What is your question?

97 Buick century loses power when slowing down and trying to gas back up I already changed coils wires spark plugs the pcv some vacuum hoses oil change new alternator new head gasket rebuilt transmission changed harmonic balancer new valve cover gaskets and it runs good at fast speeds but when I slow down and try to speed back up it loses power my speedometer is also jumpy I know I need to change the speed sensor but could that be my problem with my car losing power

What makes this problem better or worse?

It’s sporadic

How long has your 1997 Buick Century had this problem?

One month