whatever the engine accessory (need to find the right belt diagram) thats top right on a 2000 4.2 Essex w/ ac seems to be partially seized?

anyone know 1) what the accessory is? and
2) is it normal that the part stops on regular intervals and then starts again with the belt slipping over the pulley? Its like it seizes for 10 seconds, then spins with the belt for 30 - 40 seconds, then stops for 10 s. . . .
is the part partially seized?

 

It may be the A/C compressor, and if so, that's normal. It has a clutch that kicks in and out.

 

if the compressor is locked up, when the clutch engages the belt will squeal and slide over it.

but yes it sounds like normal AC compressor operation. To verify that its AC you could turn all AC off and see if it still locks and unlocks (which it shouldnt with AC off)

 

00 essex?

Sounds like your describing the A/C unit under normal operating conditions, other that what visually might look like the pulley has stopped spinning and the belt is slipping... is actually the clutch engaging and disengaging. Which still leaves the outer ring spinning. If the belt was slipping, there would be some very loud squealing.
The A/C on my 99 5.4L is bottom left, so I'm not sure what would be top right.

 

A/C on the V6's is the top right.