Third cylinder missing until RPM’s raised

This truck has been one heck of a work horse since I’ve had it, about six years. Then I was helping a friend move some things, and while we were on the road (driving some what uphill) it started loosing power and then, missing badly in the third cylinder. But without a load I can drive it and once the RPM’s get higher it’ll smooth out, then when they start dropping it starts missing again and popping. Any ideas?