Maglite 4 D Cell brightness fluctuates

I recently purchased a Maglite 4 D cell flashlight to replace one that had stopped working. I have a few smaller LED flashlights that I also use, but I've always enjoyed the big Maglite for its weight, durability, and optional use as a weapon in a pinch.

The new flashlight came with Mag-Num Star II Xenon bulbs. I wish I'd known this before purchasing a bunch of the Xenon bulbs thinking I would need to replace the stock bulb in the Maglite.

Anyway, enough chit-chat and on to the issue at hand.

I put 4 recently purchased alkaline D-cell batteries into the new flashlight and used the light on and off for about 90 minutes (not constantly on). At one point, I noticed that the flashlight started to dim significantly as if the batteries were dying. I moved the flashlight to get a better look, and the light returned to its expected brightness. Since then, the Maglite's brightness seems to fluctuate randomly. I can turn on the light, tilt it to a certain angle, and see the light dim to the point of almost going out. Moving to the flashlight to another angle will correct the problem, and the flashlight will return to its normal intensity. I've checked to make sure that the bottom cap was on securely and that the bottom spring wasn't loose.
There's no particular angle that causes the Maglite to go dim, and there's no particular angle that fixes the issue. Adjusting the beam diameter has no effect on the brightness issue.

Again, these are new batteries in a new flashlight. I tried installing 4 different D-cell batteries, and I haven't seen any fluctuation in the brightness yet (but I've only had the new batteries in for about 45 minutes). My first instinct is that there's a bad cell in one or or more of the original 4 batteries, and that's providing inconsistent power output. Perhaps moving the flashlight around jars the battery and temporarily fixes the issue.

Before I go back to the store and tell them they sold me a bad pack of batteries, could there be something else going on here? I've had this type of Maglite in the past, and I've never seen this issue before. Typically I can go 6 months or more before changing the batteries in my Maglite if I'm using it frequently. If I'm using it sparingly, I've had batteries last much longer. I realize that with the alkaline batteries, the flashlight's brightness will eventually fade over time. However, 90 minutes of infrequent use shouldn't cause the batteries to already be drained. And I still can't explain why changing the angle of the flashlight seems to rectify the problem. There's no consistency in the positions of the light that cause it to dim or that subsequently fix the issue.

Any suggestions would be appreciated.

Thanks in advance.

tighten the setscrew in the switch too.pull off the button and its down in the hole.

What brand of batteries? Also, just because you just bought them doesn't mean they are new. Sometimes stores have them on the shelves for months or years in some cases.

Try a different set of batteries before you start to tinker with it.
Different brand from a different store.

I've found that the focusing mechanism can affect the power going to the bulb. I have a 3D Mag with a MagLED in it that will flicker when I focus it.

Remove the head and reflector. Look at the little tower containing the bulb, there's a little spring loaded button on the side that the bottom of the reflector presses down to focus the light. Turn the light on and use a screwdriver or your finger to push that button down. Move it in and out a couple times and see if the bulb flickers when you do that. If it does, clean the contacts in the head, the spring under the bulb, and wherever else you can get to depending on your ability to disassemble and reassemble the head. I wouldn't rule out removing the batts and bulb and shooting some contact cleaner in there.