[v8volvo]

Most likely the rear belt tension is being set too tight (or perhaps way too loose).

If the pump were locking up, then you would be risking skipping teeth or breaking the front timing belt, which would be a disaster.

Too much tension on the rear belt will break it in short order. The rear belt should be relatively "loose" -- not so loose that it whips when the engine is idling, but you should be able to feel significant slack in it when the engine is stopped.

Adjusting tension is not the easiest but doable. 6 fuel lines and 5 fasteners hold it in place. First, remove the rear timing belt cover if not already done. Loosen all 6 injection line fittings at pump and at injectors; loosen two 13mm bolts just behind pump head anchoring rear of pump (part pointing towards the front of the car) to block near vacuum pump; loosen two 17mm bolts directly behind injection pump anchoring pump bracket to block (hard to reach, can only do with 17mm open end wrench, may need to completely remove injection lines if bolts are not coming loose easily); and finally, loosen the lower 17mm bolt under the injection pump anchoring pump bracket to block. This is the fastener that is easiest to reach (17mm socket and 6" extension), so it is the one to use for your adjustment. With rear cam sprocket loose from cam (need special tools 9995199 and 9995201 or equivalent substitutes to manipulate sprocket, assume you already have something suitable for this purpose if you have replaced the IP belt in the past), slip the IP belt on and adjust your tension, then use that lower 17mm bolt to hold the pump in place while you try various pump height settings. It takes some trial and error to find a good spot, but the belt should *not* have any tautness to it after you are done.

Put on belt #3 with a different tension setting and see if that helps. If it still breaks, then you need to stop driving the car until you get the IP fixed -- if a sticky IP breaks the front belt and takes out your cylinder head you will not be happy. My money is on belt tension, though.