Our other truck is a mostly stock 98 12v 2500 4x4 5speed. It has over 350k miles on it at this point and dosent seem as fast as it used to be. Also when it was newer (less then maybe 200k miles?) if you hadnt hit it hard for a while and nailed it in 3rd gear it would let out a healthy puff of smoke. Dosent smoke at all anymore. Runs great otherwise and no signs of any wear. Its on its 2nd or third lift pump, has had the timeing set to 16degrees, 4" exhaust and AFE proguard 7 mega cannon. Thats it. Any ideas as to what to check for a cause? fuel pressure seems to stick around 20 or just above at all times.