1981 Buick Riviera Change Heater Core Tranny sticks when hot?

Looked at a used 81 Riviera 307 4bbl, automatic and was told the tranny sticks/ won%26#039;t shift when hot. Is it shot? It ran nice with good power for 10 minute test drive. Heater core hard to change? Lots of labour involved? Any info will help. Thank you.|||wont shift when it,s hot? well it sounds like a weak pump to me. i worked on a pontiac and it acted similar to that 1 and it was the front pump. heater core? you have to get to that behind the glove box and heater ducts. as for the heater core it would,nt be that bad but knowing the transmission is,nt changing gears when it,s hot you could be getting into some money there. another thing it could be is the modulator valve on the side.|||changing the core is not a huge problem. the sticky transmission is prolly caused by a bad spring or valve in th valve body of the transmission. failry easy fix either way . you should check the tranny fluid of course to see if it burnt or really dirty before you buy though because this could mean more work than you are ready for.