[v8volvo]

Belt shouldn't stretch at all. If timing were to change, your big symptom would be poor starting and poor running after a cold start. Since the belt's length doesn't change, the only way for the timing to be altered is if the rear camshaft pulley slips, the injection pump shifts on its mount, or the belt jumps a tooth. I doubt any of those happened: in the first case, once the pulley started to slip, it wouldn't slip a little then stop and hold position again; it would continue slipping and very quickly the timing would be too far off for the engine to run. In the second case, if the pump were loose enough to move, the engine would vibrate horribly and make noises, and in short order all the hard fuel lines (injector delivery pipes, to use the correct name) would fracture and disable the car. The lines are brittle and sensitive to pump vibration and movement, and they don't last long when those things are happening. If you've been able to keep driving it for months, then the pump is quite certainly not jumping around. And in the last case, of the rear belt skipping a tooth, the car would immediately either be exceedingly difficult to start and keep running, or not able to start at all. That large of a timing change would have a drastic effect that would render the car undriveable. And as a fourth item of note, a change in injection timing, unless very extreme, should have no noticeable effect on engine idle speed, since the injected quantity is not altered. For all of these reasons, I doubt your timing has drifted.

More likely, jbg's suggestion to check for fuel restriction is where you should be looking first. Checking for fuel leaks is not too meaningful on these cars, because the entire fuel system up to the injection pump inlet is under negative pressure -- any "leaks" would not manifest as leaks of fuel to the outside, but instead as leaks of air to the inside. So, it is true that a loose fuel filter or faulty fuel line connection could leak air in and cause a runnability issue. However, it's much more likely that your filter is simply plugged. For me, the first step in any diesel diagnosis of any kind that includes a low power symptom is to replace the fuel filter. Fresh, high-quality filter prefilled with clean fuel, odds are good will solve your problem right off the bat.

If not, the only other thing that occurs to me is that the jam nut on the injection pump fuel quantity/metering screw (sometimes called the "smoke screw") could have been loose and allowed the screw to back off slightly. There are not many ways for idle speed on a VE pump diesel engine to change. In fact, as far as I am aware, there are really only three: fuel restriction/air ingress (most likely), quantity screw position change, or low idle stop screw position change. Of those three, only the first two would also affect fuel consumption and power output; the third would affect idle speed only.

Put a new filter on and let us know what happens.