1997 Firebird help please

Recently started working on my little project that started easy but I'm confused now. It ran like it was something wrong with the engine so I bought it from a guy really cheap. Come to find out it was just missing so I did a tune up all new plugs and wires. now it starts and runs perfect can even rev nice. But when you put it in gear it cuts off whether it's 1st or reverse. Its a 5 speed manual if that helps at all. But the weirdest part is It would drive fine anywhere before the tune up now it won't move but will run. Could it be the coil packs? Or what? Please help!!