95 c1500 5.7 no takeoff power

[perry]

When i first owned my truck it could break loose when stopped just by flooring it, but now about 7 months later it has no power and will barley spin while doing a power brake. It does have a used 4l60e transmission that i just put in it but i wouldnt think thats the problem. It doesnt bog ever runs great but just no power or quick acceleration any more and some times when im barly pressing the gas in gear the rpms will kind of jump up and down a little bit and no mechanic ive talked to knows why im sure someone on here could help, and im not sure if it has a vacum leak, what do yall think?