Brief combo run down:

LS 5.3, newly overhauled TH400, RMVB, new clutches shift kit etc.
New 10" PTC converter.
3.73 gear 27" tire.
Trans temp probe at bottom of pan.

Went to get the car tagged today and the trans fluid started heating up after about 15 minutes of cruising. Cruising 40ish around 1900. Everything was fine when fluid was cold. Firm shifts and all. Trans temp started crawling up to 150-160* then started slipping with a slight whining noise. When it hit 170 it slipped so bad it was undriveable and whining noise was worse but only noticable in 1st, 2nd. Shifting at this point could not be felt.

Once cooled down and it was fine again. Never had a trans do this. New PTC converter and newly overhauled TH400. Had no time to work on it or drop fluid yet, but I’m sure the trans will need to come out either way.

Another odd issue. My converter was spec'd to stall around 4k with my combo. Under 140* or so I noticed the trans is overly grabby at low RPM? At 800 rpm idle the car trys to lurch forward with the cam lope. If I take my foot off the brake the car takes off pretty quickly. Most "4k" converters I've used don't do diddly at idle. This one seems super grabby. (may be trans related too, I don't know)

Any ideas or things to try before pulling it? Anything I can try while it's installed to diagnose the problem?

Thanks