Intermattent squeel & slugginshness for 2-3 shift on 727

[69RTSEV88]

I have been driving my 69 Charger around for a few weeks now after finishing its 2 year makeover.  Due to budget limits, I did not rebuild the 727 because it seemed to shift and work just fine before I took the car apart.  The other day while on a rather long drive (about 60 miles) the tranny started to make a squeal when shifting from 2nd to 3rd, also the shift was a little sluggish, (not a slip, just not a crisp as usual).  It did this maybe 3-4 times in a row, and then the problem went away and it went back to shifting perfectly.  Just today I did the same drive again, and the same exact thing happened, 3 or 4 squealing shifts, and it squealed when I took it out of park and went into reverse.  Then everything went back to normal.  I noticed that while having the problem, if I manually shifted from 2 to 3 and just slightly backed off the throttle a bit that it shifted fine.  The car is a 69 R/T, stock 440 with a stock 727 with a B & M shift improver kit, stock converter, 3.23 gears.  I have changed the fluid & filter, fluid is Type F.  1-2 shift is always perfect.  Is it a possible heat issue, as it only does it when its hot, and I'm not running an extra cooler? Or maybe something much worse is going on.
Thanks

[John_Kunkel]

The squeal is usually caused by a rapidly cycling pressure regulator valve, more of an annoyance than a real problem. It's not usually accompanied by any operational difficulty but I suppose the cycling valve could cause a pressure drop that accounts for the lazy shift.

The factory recommended fix is to replace the whole valve body with new. I'd just ignore it.

[mikepmcs]

Take a different route next time.

[John_Kunkel]

After rereading the OP I had another thought (uh-oh) 

Since the front clutch is involved in both 3rd gear and Reverse and the squeal occurs in both gears when it's warmed up, it could be leaky front clutch seals; high pressure fluid leaks often make a squealing noise. If the seals are hardened from age, an additive like Lucas or Trans-X might rejuvenate them.