87 Iroc Idle problem, help, advice??

I have a 1987 Iroc 5.7 350. When I start my car in the mornings it fires right up but then seems to have a hesitation to the idle as if something is trying to choke the motor out. Sometimes it will turn over, rough idle a few times and die and sometimes it will not. Thinking it was the fuel filter I had it changed out but it has not helped. After the car has warmed up I can press down on the gas as it sits in park and feel and hear the hesitation between 600 to 1000rpms, anything above this range and it seems fine and I can not feel a hesitation but when the idle returns to around 600 I can sense something is not right. On the road the car runs fine without any missing and only seems to be having a problem with the idle when it is in park. I had a friend of mine who is a mechanic hook it up to check for any error codes and it showed nothing. Can a sensor be bad without showing an error code? Could it be the sparkplugs? Any help or advice would be appreciated, thanks for your time.

my buddy had and 89 iroc doing the same thing he replaced the tps throttle positioning sensor and now its fine . try that hope it works

Can a sensor be bad without showing an error code?

Yes, it could.