My 400 did exactly the same thing after a cam and head change. when idling hot in gear at a stop sign or such the light would flicker. I don't like that and eventually bit the bullet and rebuilt the old girl.
If you are doing a full rebuild, check out TMeyers site about cam bearings and the oil gallery mod. piece of cake and have a chat with your tutors about it but it all makes sense to me and I believe worth the effort. I had my crank in and checked after a few dramas, otherwise I would have done the bearings and mods myself. I had an @#$ of a time with that spring. If there is a better way I truly would like to know!! lightly knock down the plug/cap that retains and covers the spring. Remove the split pin (cotter pin whatever you call it) and throw it away as you will get a new one to replace it. then using a generous size soft faced (nylon would be good)you have to hit the oil pump while holding it in your hand. Now this is a pain as I cant remember which way I hit it as I did both till I found the way that works, either straight above the cap or the complete opposite on the other side of the pump. The inertia of the blow works on the spring and it pushes on the cap. You have to hit it quite firmly and be careful as it gets close and I would use a rag so it not only doesnt bugger off accross the shop or cut your hand. Re-installation is a lot easier, press it back in with a vice and 3/8" or 1/2" socket that will fit in the bore and remember to replace that split pin. If you've done mechanical work before you can sometimes get away with a used one if it is actually ok, but for the sake of 10c another one is the way to go. They are only mild steel so anything the right diameter will work in a pinch but the split pin is easier to work with.
Try here to have a look and then summit/jegs possible for p/a:
Moroso : Category Display