b16a problem on crx, engine problem

my car sputers once in awhile and when i reach 3.000 rpm and leave it there the car tugs a few times even in 4.000 rpm but when i punch in the trotle and revs up it doesnt lose any power, wel maybe a litle. and when the car does this i punch it all the way up to 8.200 rpm and like at 7.000 rpm the car smokes alittle black, but when the problem is not there the car goes all the way up to 8.200 and no smoke. my rpm needle is at 1.200 at idle and when the problem starts it drops down to 1.000-900 rpm. the problem is intermitent. what could it be??