Engine starts missing after it warms up

My car starts missing after it warms up. When i'm stopped and leave it in drive the engine will drop about 200rpm then rise back up to 650rpm quickly and it does it every few seconds and if I put it in neutral or park it may do it once or not at all, when I take off it hesitates between 2000 to 3000rpm, I recently replaced the spark plugs thinking that will solve it. The coils (there is one for each cylinder so 6 of them) seem to be in good condition and they weren't hot when removing them (i had to drive it about 15 minutes to a friends house) to put the new plugs in and no sign of any cracks and the wires leading to them are all in good order. I replaced the fuel filter at about 145000 miles and air filter and oil change and oil filter at 155000. I noticed that there is a split in the blowby hose that goes between the two cam covers and the air cleaner, so please any ideas of what the problem is. It's been doing it since about 150000 miles but it doesn't seem to be getting worse