I have 97 K1500 with 258K miles and I have low power with bucking/surging under load. I don't believe it is a clogged cat since at times it runs with normal power. There are no engine codes. Idles well. I haven't checked the fuel pressure yet but will do today, what should the pressure be at idle and at any other rpm I should check? Let me know if these other things could cause the same conditions:

Weak coil

Lazy/bad O2 sensor

Timing chain

Vacuum leak (best way to check?)