just purchased an 87 camaro 2.8 auto for a winter runner... has a few issues

it just got a tune up (plugs, wires, air filters...)

cold start the car starts just fine. the vehicle hardly misses at all. park ide is around 1200 rpm. once the car warms up tho idle becomes less than 1000, and misses more. and when i attempt to start the car after its warmed up, its a son of a gun to start... it cranks and cranks until i give it a little gas then itll start up.. the car misses more as well... i also noticed a check engine light will pop on after about 20 minutes of constant driving and notice a harder brake pedal at that point. my transmission also shifts a little goofy. it will not downshift out of say drive and overdrive unless i usually have the pedal pretty far down. plus overdrive likes to shift in and out too much at 45-65 mph. are all these related problems? i disconnected the iac and the car starts much easier but will run about 2000 rpm idle...