What is causing my truck to break into a shutter while under load. If towing a load or running down the highway empty and climbing a long grade the truck will start to shutter after trying to maintain 65 or so up the grade. The shutter will get bad enough that everything will shake and you can not accelerate at all. If I let off of the gas for about 10 to 20 seconds it will relax and I can take off again.??? It has a brand new tune up, new cats, new fuel injection (spider), and I had the local tranny shop run the scanner on it while I drove it and it showed NO slippage on the torque converter. We did notice that the exhaust seems to have a slight sucking action maybe bad valve? timing? not sure. Does seem weak on power. 150,000 miles Any ideas?