We do some service on a 2000 Kodiak 6500, Cat 3126, Allison, 30,000 miles, but it's a bucket truck so a lot of idle time. They have recently had an engine issue and some other things we've already repaired. All the fluids are fine including fresh oil change (was acting up before the service). Fuel filter is new, air filter is good. It starts and runs fine and runs completely fine while cold or during warm up, but after fully warmed up (at least 10+ miles) it will loose power and develops a miss when subjected to any kind of decent throttle load. As soon as you back off it runs fine. It will run fine at light throttle too (which isn't very effective as underpowered as it is). There is no certain point that it acts up. Sometimes it can run fine up to 2600 (maybe even into 2nd and 3rd gear) and then start missing and drop off. Other times it can run OK for a short time, but then miss and not climb higher than 2000. And still other times it can start acting up shortly after you start accelerating and just not pull above 30-35 mph (maybe 1500 rpm). The check engine light is not on during any of this. Would anyone have a suggestion please? Thanks.