LinuxQuestions.org - Grub only boots on 2nd attempt - menu reappears a 2nd time, then it boots.

I've dual booted Mint 18.1 w/ Vista on a desktop several yrs. Works great. But the OSes are on different HDDs.

Now, an old laptop needs a new, safe OS. It has 2 internal HDDs, but one is ~ 120 GB, has some stored stuff AND it's an Hitachi (I'm surprised it's lasted this long).
So I put Mint /w Vista on same WD Black disk - normally OK. The Linux / root partition is in an extended volume, but that didn't affect Mint Mate. Mate dual booted fine - same exact setup as (the later) XFCE, but Mate used too much RAM for the laptop.

Decided to try Mint xfce 19.2. Runs great - the problem is in the dual booting.

I formatted the Mint Mate partitions & reformat to ext4, EXCEPT my separate /home partition. May have messed up, or not.

There's also separate /var, /tmp partitions - reformatted. Didn't change partition sizes, starting sectors (afaIk); used same mount points in same partitions that Mate used.

Now, I'm having a couple of moderate issues w/ XFCE.
Did I mess up by not backing up personal docs in /home (basically nothing), then formatting /home to get rid of any Mate / Mint 19.3 config files, before installing a different Mint version & diff desktop?

Grub loader has a weird issue. I'd appreciate any suggestions on this one.
FIRST time I boot, it shows the dual boot menu & Mint is in 1st position. BUT... whether I click / enter on Mint or let Grub timer start the default Mint, the Grub menu goes away, then the SAME menu reappears in about 8 - 10 sec +/-. Like nothing happened.

The 2nd boot attempt - whether click the default entry, or let Grub start it - it boots perfectly. That's why I asked about not formatting all config files from /home.
It's not doing some "recovery", cause several times I've manually clicked the highlighted 1st OS - still only boots on 2nd attempt.

If it works fine on the 2nd attempt - every time, it knows where everything is, with right settings / values. I've repeated this enough to see it's not a fluke. Unless there's a setting, "Only boot on 2nd attempt," I'm stumped. I'm thinking I can run every Grub test command, but if it works on the 2nd attempt, there aren't any Grub file errors. Maybe a bug in Mint's implementation of Grub or other problem. The BIOS system time was off by 1 hr. I corrected it, but didn't help yet.

Maybe it discovers more info in short span, between the 1st & 2nd grub menus. :thumbsup: I've read the Grub info manual, found all grub files in proper path. Nothing obviously wrong, discussed in dozens of grub articles.

Other odd things: In file manager, I see 2 Desktop folders - neither is at the top, under the user's home folder. Menu has shortcuts for both Thunar & Caja (both may not work, but).

And file mgr - side bar always shows UNmounted, non-Linux partitions w/ labels (aka, Places), between /home config folders & File System. Not at the bottom of tree. There is no icon or setting I find to toggle off showing Places in the file mgr Tree.

In file manager Shortcut view, labels for Devices & Places are switched from where they should be. It's a mess.
I guess Samba is working - can mount / unmount any partition on both HDDs.
Thanks.