I've got a 1999 Nissan Altima 5 speed with around 112,000 km on it.
It's a great car, but has always had one annoying quirk.

When I'm coasting in gear, with the engine speed between 1500 and 2000 rpm,
there's a sort of double jolt as the power comes on when I ease into the gas
as slowly as I can.  It's about twice as bad when the car's cold, until the
engine warms up completely, and it also seems to be worse when rolling
downhill or on a level piece of road, versus when going uphill.  There's
also a corresponding harshness when I release the gas around the same revs.

I wonder about the motor mounts because the engine will sort of bounce
forward and back when passing a certain point in the rev range when I blip
the throttle while under the hood.  This might be a momentary miss instead
however -- the car's always had a slight hesitation off idle when I blip the
throttle, so maybe this is related to that.

There's another behavior that might be causing the jolt; when I'm stopped in
neutral, I can't hold the revs between around 1200 and 2000 RPM; as I pass
1200 RPM the engine suddenly revs up to 2000 RPM and hangs there until I
release the pedal slightly, at which point it returns to around 1200.
I can see how this would cause the exact symptoms I have, but I'm not sure
if this revving past 1200 RPM is normal, meaning there's something else
wrong (engine mounts?) or if this is what I should be trying to get to the
bottom of.  If I had to guess, I would say that this is the underlying cause
of the problem, as it's basically as if the throttle is "off" until I cross
the point where it wants to rev to 2000 RPM, at which point I get the jolt.

Can anyone offer any thoughts/ideas on what might be going on here?  With
the cold weather it's a fair bit more noticeable now and pretty
uncomfortable until the car's warmed up.

For the record, I have tested the adjustment on the TPS and it appears to be
bang on.  There's also a throttle on/off switch on the throttle shaft, which
is adjusted in tandem with the TPS and seems fine, although disconnecting
this has no affect on the car at all, which almost makes me wonder if it's
not communicating back to the ECU.

Thanks in advance,

John