69 Mach 1 FMX shifting problems!!! HELP!!!

[PCGamer]

Purchased car recently with a fresh motor\tranny rebuild. Have receipts and have previously spoke with shops that have done the work...

Motor (351W) has been rebuilt as if it were a new long block.. Has edelbrock 650 carb, with a performer intake. Tranny (other than torque convertor) has been completely rebuilt i.e. front servo, clutches, bands, seals, etc. etc. etc.....

Both motor and tranny have about 900 miles on them...

Within the first 20 mins. or so with a cool engine/trannsmission the transmission downshifts and upshifts correctly in all gears regardless of accelerator pressure even if it is floored....

Once things are heated up (after driving for about 20 min. or so) this is when things start... The tranny shift point between 2nd to 3rd (ONLY) is not occuring at WIDE open throttle (FLOORED) until I let off on the throttle and engine rpms DROP to around 3400 then it will finally shift into 3rd gear. Again this is ONLY happening once the tranny is heated up with the accelerator FLOOED... Otherwise even when heated up the tranny will shift fine except once again only when the accelerator is FLOORED...

I've replaced the vacuum modulator and checked the fluid. Unless the modulator is bad I'm not sure what this. The tranny is basically brand new with low low miles... The car was purchased out of state so taking it back to where the work was done is NOT an option.. Im hoping its nothing expensive...

Im thinking maybe its a vacuum issue but again while only once the engine is hot and the pedal is floored???

Please Help..

PC....