2002 4.7L Dorango rocker arm problem?

I have a 2002 Durango, the #3 Exhaust rocker arm keeps coming off, it was took back to car lot and they installed it. It lasted a day, now the shop has replaced rocker, and lifter, changed oil to 5W-30 and tried another camshaft. It starts and runs but by the time you get a block it starts again. The old cam was worn on the same #3 cylinder but we think driving it with arm off caused the damage. The spring feels still when we try pushing it down with a prybar, can a spring be changed on the vehicle?

21sprint, yes the valve spring can be changed still on the vehicle. Odd problem.

21sprint, yes the valve spring can be changed still on the vehicle. Odd problem.

Not just odd but pretty hard to repair without buying the valve spring tool needed. The cam had a bad lobe on same cyl. so I tried that 1st but still kicked off rocker. I put air on the cyl. and valve spring was weak, but that wasn't the problem, the valve stem was bent. I was told not to try and staighen it but did it anyway, runs fine now. Lesson learned..:)