There may be enough rust in the galvanized nipples and elbow that it's backing up to the showerhead. A typical shower valve has some internal flow limiting - with two valves that can probably open fully to the supply, it may never work right since you're overwhelming the outlet with more volume than it can accept. You might want to try changing those to brass or copper, but it probably won't fix it. You may need to add a shutoff valve on the riser. On a totally different tack, though, you really should consider replacing that old valve set with a new, code approved anti-scald valve. That was okay when used for the tub only, but does not meet code when you change things to a tub/shower.