Bucking,misfiring after warmed up

[purplemacaw]

1991 Z-28 Camaro bucks & misfires & shakes after it warms up! Some of the fuel injectors were shorting out & car wouldn't run & mechanic changed all the injectors & now it runs like a bear until you warm it up just a little & if you turn it off , when you start it up a few minutes later, it bucks & shakes. When you try to drive it again it runs terrible until you park it for many hours, then when it has completely cooled it will run like a bear again!! The hotter the car gets the worse it runs! Mechanic has checked everything & says everything else seems to be ok & can't find what is wrong! Service engine light does Not come on! Any ideas? Now a week later it starts to misfire after about 15 minutes of driving without getting it very warmed up or turnig it off! Getting worse! Give it gas & it misfires & bucks & holds back on acceleration. Shakes at idle. HELP!! Any ideas??