If running right, you should be able to do 80+ although it'll be wound out at that point.
First thing to do is give it a couple of treatments with auto rx ( www.auto-rx.com ) read the online instructions for diesels. this stuf is costy, but it really works. chances are your rings are pretty gummed up or downright stuck. it's very common in these things, the auto rx wil take care of that

Sounds like you may ned a new thermostat, if you're in CA, get the lowest temp one you can find, the pars stores usually list one for an Audi 5000 diesel that is a lower temp than the ones listed for the volvo .
If the engine is running cold, the idle will stay high, there is a cold start idle control setup on the injection pump.

Do you know when the timing belt was replaced last ( may be written on the valev cover, or a sticker in the engine compartment ) 50,000 miles max on that belt or it's all over.

You might want to get on this email list for additional support http://autos.groups.yahoo.com/group/volvo_diesels/

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-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '82 Mercedes 300SD