Gearbox (clutch?) trouble + engine question

[Yonzie]

Hey guys.

When I start up my car, the idle is at 2200RPM after it heats up a bit, it falls to 1000RPM... I've been told this is normal, but it's annoying... Is there any way to change this other than swapping out the engine for a newer one like the 20v?

Now, on to the meat of my troubles:
When the engine is cold, the gear shifter is in neutral, and I release the cluch, it's like the car very very slowly creeps forward. If I release the clutch quickly, there's an audible "thunk" sound. After a bit of warmup, when the engine is at maybe 2000RPM, I can make it fall to 1500RPM by doing the same.
This is a recent development over the last ~150km (mostly city driving).

The car has 240k km on the odometer, and according the the previous owner, the engine is the original one. Although I didn't think to ask him, I suspect the same is true of the gearbox.

Any idea what's wrong and what I need to do to fix it?