1994 Ranger question

1994 Ranger XLT 3.0V6, 4x4, no a/c, auto trans, 90k. Has been a great truck overall. Jan 2007 had to put in a new radiator. Service shop had to go thru 3 different radiators before finding one that wasn't supposedly defective. Truck was fine until this Sept, when the radiator went out again. Brought it in under warranty and had a new radiator put in. Now the truck seems like it possibly has some kind of blockage in the radiator lines? Shop says they've never seen anything like this before. Running temp is fine, thermostat is fine, radiator cap is fine. Not throwing any codes either. The truck kind of audibly "pulses" when running. When you open the hood and look at the lines/hoses, you can visually see the hoses pulsing. They think there might be some kind of blockage somewhere. If not that, they think the water pump is going out again (had a new one put in only last year), or possibly engine is going out. As I said, they've never seen anything like this before. I have a hard time believing the engine is shot. I stay on top of all regular maintenance, and this problem started when the latest radiator was installed. HELP! Anything you can tell me? Or point me in the right direction? Thanks.