I'm not a diesel mechanic or anything, but I had the opportunity to troubleshoot a bajillion fuel problems when I swapped the diesel into my sami, and then some more when I did all of the injection pump mods and the turbo swap...and I've done injector and injection pump swaps and a fair amount of troubleshooting on a few pieces of heavy equipment and pickups. It's been my experience that 95% of the problems with diesel engines are fuel related. Also, let me preface this by saying that I'm not familiar with the Renault 1.9L diesel; I've never even seen one. So I don't know the specifics of your particular engine. But the troubleshooting and fuel system info does apply to pretty much every diesel engine out there.
Basically your fuel system works like this:
The diesel in your tank is pushed from the tank by the lift pump (not all diesels have these, but most do). The lift pump is a small, low pressure fuel pump that provides a positive pressure to the injection pump to keep it from working too hard by "sucking" the fuel all the way from the fuel tank. This pump also helps protect against air bubbles by providing a positive pressure in your fuel system...if your lift pump is bad then the smallest loose connection or pinhole leak will introduce air into the lines as the injection pump "sucks" the fuel from the fuel tank. These air bubbles can be too small to see through fuel lines...enough of them can aerate the diesel fuel and cause the engine to run poorly or not at all. Assuming your fuel lines are tight, your lift pump is working properly, and no air is being introduced into the fuel system, the next component in the fuel path is the fuel filter; if it becomes clogged (which happens quite often with poor fuel or after switching to biodiesel) it will starve the injection pump of fuel when it needs it (at higher loads and RPM's). The symptom of this is a loss of power or "dead pedal" when accelerating.
Next is the injection pump itself; it takes the fuel from the fuel line and pressurizes it to a very high pressure to send through the injectors at specific times (this is how a diesel engine is "timed"). The high pressure is why the fuel lines to the injectors are metal. Genrally, injection pumps are not servicable by even skilled mechanics. Although the timing, fuel pressures, and idle speed are adjustable, the inner workings of the injection pump are best left to diesel specialists. If it is bad, your engine can also show a loss of power, and even not run at all.
The injection pumps are by far the most expensive component to replace, both in cost of parts and labor. So, I'd start by eliminating everything else. The best thing to do would be to get a manual for this engine and follow the procedure for doing a volume and pressure test for the lift pump (I'm assuming you have a lift or transfer, or whatever renault calls it pump either in or somewhere near your fuel tank). Basically this involves removing the fuel inlet line from the injection pump and running the lift pump for a specified amount of time into a cup. If the filter is good and the pump is working properly, you'll get the correct amount of fuel into the cup (again though, I can't tell you what this volume is as I don't have any specs for a Renault 1.9L diesel). The pressure is checked with a fuel pressure gauge.
Although watching the fuel line while driving the vehicle might tell you something, my guess is that you're not going to see anything but fuel moving through there...and there's no way to gauge how fast or how much fuel is moving, and if that's enough for the injection pump, or if the lift pump is working.
