Shifty Buziness

[1970amx]

My truck is equipped with the 47rh automatic. it has been upgraded with a aftermarket torque converter which is blue and dont know anything else about it. It also has a towing valve body and shifts a little firmer than stock. On the shift from 2nd to 3rd, particularly when cold is very ruff and hesitant. when the truck is cold and sat overnight, it will run clear up to 2500 rpm in second and never shift. U can just hold it there at that rpm and it wont shift. Even if u let off the throttle it stays in second and will eventually shift into 3rd. After truck is warm under any hard or harder acceleration 2nd to third is still hesitant and hard. under full throttle 1st to 2nd is normal, just right. 2nd to 3rd is awful though. it will redilne for a few seconds (2-4) before it shifts hard into 3rd. my foot would have been to the floor the whole time. shifting into overdrive is fine also. Anyone know what exactlly is going on in this slushbox of mine? any help is greatly appreciated.

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1995 2500 SLT 12valve auto 4X4: regular cab longbed :factory 4.10's and looking for bigger tires; #6 fuel plate and gutted hfc housing, 4" downpipe and 5" out a single mitre, towing/performance valvebody and aftermarket(blue)??? lockup converter.