Test for A/C clutch 'grip'?

I've got a 98 Suburban with the rear air. The A/C was last repaired in 06 with a new compressor etc and had been performing nicely before being parked 5-6 weeks ago.

While checking the pressures, I noticed to clutch stutter a few times before engaging. Afterwords it appeared fine but the performance is poor.
How can I test it, or what do I look for to decide if it's just the clutch plate needing replacement rather then the compressor? I'm assuming there is not a 'variable' type of engagement control that's possibly defective. Is this wrong?

low 45 high 175 with rear on, 50-200 front only.
Cooling system is great, I still used a nice aux fan and even a garden hose to be sure.
Outside temp is 100, the vent temp 60 or so but did pull down closer to 50 once on a lengthy drive. Normally, I'll get 28 low and 280ish high, and a vent temp at 40 even when it's hot outside.

I realize the compressor could be giving out, if it wasn't for that goofy clutch stutter I wouldn't suspect it.
Thanks!

My pressures are low 45, high 175 with rear air on, 50-200 front only.
Vent temp of 50-55 degrees
With the cooling sytem working properly, is this simply an indication of a failing compressor?
My usual pressures were 28/280 with a vent temp of 40 with the same outside temp as now.
Am I correct in only adding freon (without knowing whats in there) if the low pressure is also low?
Thanks