first-time poster, here... with apologies in advance for anything stupid or ignorant that i might ask or say... here's what i've got, what i've done and what i've tried: 1992 Chevy Silverado 5.7-liter, 350 tbi Acquired vehicle in May, 2015 170,000 on odometer (suspect actual mileage is higher from looks of engine wear...) Conditions: Starts and runs great when cold. Runs rough (from day one) when at operating temperature or hot. Won't cruise smoothly at any speed (when not accelerating or decelerating, the engine hiccups, shudders, misses). Gas mileage is very poor. (I understand this vehicle has low MPG in any condition.) Dies at stop when hot. Misses (on one cylinder, it sounds like) when hot, at idle, in gear, with A/C on. Idles better when hot if taken out of gear OR turning off A/C. The Power under load when it's hot is very weak; sputters, shudders and even dies, occasionally. Use: Began using vehicle for work tasks, pulling loaded 16' dovetail trailer, in mid-June, 2015. Runs well at beginning of day; 20 miles down the road starts to shudder and miss; at the end of the day it's limping in and I'm wondering if i'll make it home. Research, Diagnostics and Repair history: Began diagnosing and making minor repairs July 1. Took it to a shop who said I had a vacuum leak and adjusted the timing. Lost confidence in shop when truck demonstrated same behavior. Suspected fuel/fire problems. Replaced fuel filter. Replaced fuel pump relay. Replaced spark plugs. Checked wires. Talked to some smart people; consulted with a 45-year large equipment mechanic; read more stuff online; watched some videos on youtube. Replaced distributor cap. Replaced distributor rotor. Adjusted timing. Minor improvements noticed; same problems evident when hot. Suspected fuel regulation problems. Inspected injectors for spray pattern; shows even spray with adequate pressure (visually, anyway). Replaced MAP sensor. Replaced Idle Air control Valve (IAC). After replacing the IAC, the idle speed jumped 1000 rpms. Adjusted timing. Replaced temp control sensor (the one that talks to the EMC). Checked timing. Minor improvements noticed; same problems evident when hot. Replaced EGR valve. Replaced EGR vacuum solenoid. Adjusted timing again (this one done by a racing mechanic who knows what he's doing). Tested for vacuum leaks (spray). Replaced ignition coil. No change in performance. Took it to a second repair shop, recommended by one of the racing machanics, who suggested a clogged catalytic converter due to whiteness on the manifolds and/or a sticky thermostat. Replaced Thermostat. Vehicle was running at about 185 degrees before and is running about 185 degrees with new one. No change in performance. Took it to a muffler shop and had the catalytic converter tested and it's doing its job (though i do have some pipe cracks between the converter and the muffler). Today I tested compression with these results: 1-150, 3=145, 5=150, 7=160; 2=160, 4=130, 6=150, 8=100. While checking compression, all 8 spark plugs show chocolate-colored markings, indicating no lean/rich conditions. She's running _pretty good_ now, after all of this... but i still have the ORIGINAL problem/complaint: when hot under load, she misses, shudders, quakes and loses power. this must be harming my fuel mileage and makes her unreliable - the latter condition unacceptable for work use. I have NOT tested fuel pressure (tho it appears to be spraying healthy amounts from injectors), or throttle position sensor (can't see that it applies) or fuel pressure regulator. So, my question is: WTF? Ignition control module? Distributor? Intake manifold gasket? head gasket? i mean, sheesh... I appreciate your time in reviewing this information. Thanks for any guidance.