All 120v outlets and lights are not working in a detatched garage
All of your symptoms sound like a faulty Neutral connection.  Faulty neutral causes L1 to affect L2, and has no affect on 220 equipment just like you described.  

Fix that quick.  A faulty neutral can damage your equipment, and because you installed a second ground, it can harm YOU as well.  Check that the neutral wire is firmly connected, and using Noalox:



ALL aluminum connections for that matter should be protected from corrosion and thermal expansion with Noalox.

You are lucky that you have a conduit!  You have an EASY way to remove the entire length of the neutral conductor to inspect it for damage.  You should also take advantage of that conduit to pull a ground conductor through it and use that to ground your sub panel VIA your main panel.  Do not use a second ground rod at the sub.  It is not worth the risk in your case.