Hello and welcome to Just answer.com, Super mechanic here. About your vehicle.
There have been a couple of service bulletins for that type of concern. One will have to do with the PCM ground wire corroding,(which doesn't sound likely to yours, being it has been going on) and the other, ( misfire DTC under load) may be the problem. It has to do with the valve stem to guide clearance, (apparently not enough when the engines were built) Here is what they are.
Subject: SES Light and P0300 When Towing, Cruising Uphill or on hard Acceleration - kw 4.3, 5.0, 5.7 accelerate cruise cylinderhead DTC L30, L31, L35, LF6, LU3, P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307 P0308 # XXXXX - (07/23/2004)
The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.
The vehicle may exhibit a SES Light due to a P0300 and misfire. If the misfire is related to the information below, it will typically happen while cruising uphill, pulling a trailer or on hard acceleration and then stop misfiring shortly after returning to an idle. Typically, cylinders 3, 4, 5 or 6 will be the ones to experience this.
If the P0300 SI diagnostics did not isolate a concern, the following may help:
Remove the valve cover, valve springs and valve seals on the effected cylinder or cylinders. A small wire tie or rubber bands can be placed in the valve stem keeper groove to prevent the valve from falling into the cylinder. Rotate the valve while moving it up and down in the guide to see if it binds. If a binding valve is found, remove both cylinder heads and use either of the following methods to increase the stem to guide clearance:
Reassemble the cylinder heads using new valve seals.
Bulletin No.: 06-06-04-046
Date: September 12, 2006
Subject:Information on Engine Misfire MIL/SES Light Illuminated or Flashing DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0420 or P0430
Models:1999-2007 Cadillac, Chevrolet, GMC Full-Size Pickup and/or Utility Trucks
with 4.8L, 5.3L, 5.7L, 6.0L or 6.2L VORTEC GEN III, GEN IV, V-8 Engine (VINs V, C, T, Z, B, 3, M, 0, J, R, U, N, Y, K, 8 - RPOs LR4, LY2, LM7, L59, L33, LC9, LH6, LMG, LY5, L31, LQ4, LQ9, L76, LY6, L92)
with Active Fuel Management(TM) and E85 Flex Fuel
If you encounter vehicles that exhibit the above conditions, refer to SI for the appropriate DTC(s) set. If no trouble is found, the cause may be due to an ECM ground terminal that has corroded with rust over time. Inspect the main engine wiring harness ground terminal (G103) for this condition.
Remove either the nut or bolt securing the main engine wiring harness ground terminal (G103) to the right cylinder head. Refer to the above illustration to determine where the ground is located on the vehicle (1).
Remove all rust from the ground terminal, the cylinder head and the retaining nut or bolt.
Position the main engine wiring harness ground terminal and install the nut or bolt.
Tighten the retaining nut or bolt to 16 N.m (12 lb ft).
I have read the answer and the symptoms match what my truck is doing. But this is going to be a hard one to verify without tearing it down and doing the repair. Do you have any collaborative information to support the valve clearance to guide suggestion? We can find a TSB on the ground wire, but do you have a TSB number on the valve clearance recomendation?
The questions that I have are
1) why wouldn't the valves wear over time and loosen up instead of getting tighter?
2) is there a tsb on this problem, or a gm bulletin of some sort that supports your response?
3) how do the valves know that the engine is under load?
Yes. When the engine is under a load then all of the engine components will have to do their job more precisely. The bulletin for the valve guides is as follows.
Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.