There are different taper profiles but for most applications mixing is not a problem. I'd check twice that the spindle is turning smoothly and freely in the bottom bracket cups. If there is any looseness or binding you can bet the problem is in the cup and spindle interface.

If that checks out, though, make sure your chainrings are straight. Remove them and see that they lie flat on a surface you know to be flat. While you're at it you can do the same for the crank spider.

If that checks out, look at your spindle. All spindle tapers, and cranks too, for that matter, are not perfectly true. Put the rings back on and look at the crank-spindle interface. Try the drive side crank in each of the four positions and go with the one that produces the least chainring wobble.

While you're down there make sure your chain isn't bent, too.

Finally, make sure the mating surfaces on the crank and spindle are perfectly clean and dry when you do the final reassembly. Do grease the end bolts and washers, though. Crank the end bolts down hard as you can without using a cheater bar. Then go for a ride, and re-tighten.