Those RPM are about 3-10 times what the Volvo is rated for. Or do you just have an extra zero? And yes, it sounds like wear or ??? on the Martec. Diver can probably take a look at it and advise on refurb/replace prop if necessary. If these are actually in the 750-1000 RPM range, could also be due to the engine not being adequately warmed up yet.