truck runs good when cold, after it warms up (5 mile drive) it starts to act up. no power and cant get over 35 mph unless going down hill. idles fine hot or cold. i am going to change the fuel filter, but i think its the catalytic converter. 96 f150 2x4 m5od 4.9 six. any comments?

 

It could be the cat, but just to be safe, I would give it a full tune-up, change all the filters, the PCV valve, and check the timing. An exhaust can do an exhaust pressure test which will tell you whether or not it's the cat. If you have the cat replaced there, they likely won't even charge you for the test.