Has this truck ever ran satisfactorily in service since you acquired it or have you been trying to resolve this issue ever since you got it?
Has the exhaust system been checked for blockage?
We once had a Komatsu exhibit somewhat similar symptoms and that turned out to be where the partition wall (baffle) inside the muffler broke into pieces and a large piece stuck against the muffler outlet.
The engine would idle but would die if you tried to accelerate.