I gather it just randomly started doing this and you're not asking this after having either messed with the tune or rebuilt the motor....
A cylinder (or multiple cylinders) is (are) not firing. Generally speaking, it means either it's not getting enough fuel (injector clog), or it's not getting spark (wires/plugs).
Edit to slim down....
1. Get mechanics stethoscope and listen to the injectors. If they ain't clickin', they're a stickin'..... (Cheesy.... LOL)
2. If they're clicking, spray the header/manifold pipes from each cylinder with water when warm. If the water doesn't evaporate if warm or sizzle when hot, you have found which cylinder(s) isn't (or aren't) firing.
3. Inspect coils, plugs, and wires at offending cylinders if they exist.
The solution to #1 is easy. Replace. You may want to have the others bench cleaned to be sure you get even injector performance.
For #3, obviously you have to replace the offending part. If they haven't been replaced in a long while and the coils test positive, then just replace them all and try again.
However, if you're problem is because you've rebuilt the motor and are now tuning it; that's a whole 'nother ball game.