I did the same thing as Bluebird with the relays and it does help.

But back to the original question,
I don't know that the bulb is your problem. I used some (Wagner) Halogen H5001 & H5006 sealed beams on my car without any problems at all. Though I don't remember them having the big rubber cup on the back and it was many years ago now that I did that (plus the car is in deep winter storage right now and I can't get to it to check.) But, make sure that terminals inside the plastic plug are in the plug all the way, and coming close to the front of the plug. As options you could pick up a new headlight connector pigtail and see if will fit better on your headlight and the wire it into your harness if it does. Or as a temp. fix pull the wires out of the pigtail put a male end on the other end of each wire and use them as short jumper wires.
Though the simpleist may be to just cut the front of the plug off.
Lee