[Bellerophon]

If it were the tappet/nozzle, every shot will be inconsistent. You may hit a streak of good shots, but it would quickly stop working again.

Also: try loading a few BBs into your mags and then release them by hand. If they come out of the mags dirty or broken, the mags are at fault and the feed track needs to be cleaned.

I don't know what brand mags I use, I bought my MP5 as a package deal on the classifieds. As for BBs I use BBBastard .28 nonbio and Madbull .28 bios. I have no idea how good goldenball BBs are, so here are the tests I use to determine if a BB is any good:
1. Inspect the surface for mould lines, pits or lumps. The surface should be perfectly smooth and glossy in finish.
2. Crush one BB in pliers. It should squish before breaking, not shatter.
3. (not strictly relevant here, but good practice for evaluating any brand of BBs) Cut a few BBs in half and make sure there are no bubbles or gaps in the center. If there are any, these can throw off your accuracy because of the uneven weight distribution.
4. If available, use some calipers to check the diameter and tolerances of a few sample BBs, and ensure these dimensions match those on the packaging. If the packaging specifies tolerances wider than +/- 0.01mm, or worse they don't give tolerances at all, the BBs are probably crap.

Hope that helps.