[V8] Rough Engine

[Brad Hartman]

My 1990 V8 runs fine when cold, but after warming up, the car runs rough.

By rough, what I mean is when I accelerate, the engine seems to stutter. 
The car still speeds up, but you can detect a rough accel profile.

Also, when idling, it seems to lose RPMs just for a fraction of a second, 
them resume normal idle, and repeats this every few seconds.

Once the car is cooled down, and I restart, all is well, both during idle 
and full speed.  But when it gets hot, it returns to rough operation.

No Check Engine Light is on.

My engine temp gauge seems to peak out at around 95 degrees C.

Air filter is pretty new and not full of dust.

I have noticed this for about the last 3-4 weeks.  I have changed gas 
several times from diff sources, so i don't think it's gas.

My first question:  Should the engine temp gauge almost reach 100 degrees C 
(red needle is almost at 12:00 O'clock)?
It seems in the past, the hottest it would get is around 1/4 of the gauge 
reading, or around 50 degrees C.

Bad thermostat?

Second question:  Any places I should check first for source of rough 
engine?

198,000 miles on engine.

Thanks,

Brad