1996 Chevy Tahoe 136k 4WD no faster then 30MPH

My 1996 tahoe has always ran great, but just recently its had a problem where if you drive it for about a min or 2 it runs amazing no problems! then after that it shutters and wont rev up past 2k RPM but it drives and shifts so my top speed is like 30mph.. i checked plugs, they where dirty, cleaned them, wires, cap, rotor, brand new and termanials cleaned.
i was thinking cats are clogged and the exhaust pressure is building up then causeing it to misfire

OR

bad fuel pressure regulator?

hints? or suggestions?