Lets see...your not completely aware of the past history of this truck..yes??
OK this is my twist to a possible reason.
I'd say the past owner lost the head & or gasket....in the winter, or late fall...got a crankcase full of water...let it sit for a time cause it wasn't "running well"...sever cold set in...got it running...for a quick trip ran worse so they parked it til spring...maybe changed hands a couple of times...somebody replaced the head and or gasket, got it running better, but not right, then sold it. as Paul Harvey would say, "And now you know the rest of the story"