90 Grand Prix 3.1 help!

Well first things first. Car has been running a little rough as of late. As I was changing the plugs, I noticed a vacuum line was off, I think. Maybe it fell off as I was tilting the engine forward but not sure. Still runs rough though so thats besides the point. New plugs and wires, fuel filter, PCV were recent new part put on. Also, last summer I put on a ready fit catalytic converter. Just had to splice it in, but ran just fine after that. I took it to Oreilly's auto and borrowed their OBDI and found no codes.
When its at idle in park, it runs ok, seems like the exhaust has a little jitter in it. Car in drive with the break on, definitely more jittery and would stall out sometimes. Hasn't really stalled out now that I put that vacuum line on but I also put it in neutral at stop lights to prevent this so maybe it would still stall out, who knows.(If it does stall, it fires right back up) Taking off, it seems to take off just great, unless I very slowly accelerate, then it has a little jerking (hesitation) that occurs. Also, when Im at cruising speed (35 mph) and hold the gas steady, thats when I notice jerking as well. Nothing too fierce but theres definitely some 'bogging' out. I really don't know where to start being that there are no codes thrown. I have some oil leak thats over by the belt system and not sure if thats effecting anything. I know the belt gets oil on it as it splatters a line on the firewall after awhile. (sort of like a line mud on your back when you go through a puddle on a bike). Other than that, thats all I got. I really hope someone has an idea.. Thank you!

ohm check the injectors. look for right around 12ohms each. anything less and it needs to be replaced.

I agree with Tim. It sounds like you have an injector or 2 going bad since it's kinda random and more prevalent when at steady speed or light accel.