do you know exactly what code it was? a particular cylinder or was it a P0300 random misfire code?
if it is a 300 code that could be a couple of things that come to mind. Usually I see just one injector go bad, and you just had them serviced. All the secondary ignition has been replaced, so we can rule that out. so that leaves:
Unfortunately the best way to find a problem would be to hook up a scan tool and go for a ride with the trailer attached. I have had to do that to find a problem under heavy load, or I find the steepest grade possible and try to load the engine to duplicate the problem.
Hope this helps
A fellow expert just sent me a TSB if the above recommendations do not help
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 #PIP3081 - (07/23/2004) Models: . --------------------------------------------------------------------------------The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.Condition/Concern: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.Recommendation/Instructions: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. .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.Models:(96 - 02 Chevrolet Astro) and (96 - 02 GMC Safari) and (96 - 00 Old Body Style Chevrolet Suburban) and (96 - 00 Old Body Style Chevrolet Tahoe) and (96 - 02 Chevrolet Express) and (96 - 00 GMC Yukon) and (96 - 00 Chevrolet Old Body Style C/K Truck) and (96 - 00 GMC Old Body Style C/K Truck) and (96 -02 GMC Savana) and (96 - 99 P32 Cab and Chassis) and (96 - 02 Chevrolet S-10) and (99 - 02 Chevrolet Silverado with 4.3L V-6 Engines (96 - 02 GMC Sonoma) and (96 - 02 GMC Sierra with 4.3L V-6 Engines)