Thx...can you elaborate? E.g., is it likely that they are generally undertensioned? Or, some under, some over? The wheel is actually pretty true.
To expand on what highdelll said - chances are all the spokes are too loose, and that the ones you tighten are getting twisted instead of actually tightening on the threads. If the wheel is "actually pretty true" even with a couple spokes being obviously loose, then they are nowhere near tight enough. It should be going out of true (from tension imbalance) long before the spokes are obviously loose.
You want the OPPOSITE of thread lock; the wheel is old, so those nips have some corrosion in the threads, which is making it hard to properly tension the wheel. Use some penetrating lube on all the threads, tighten them all a quarter turn, then tighten the drive side another 1/4 turn, stress relieve them all, and then true the wheel by tightening only (max another 1/2 turn).