Change the Weldon site from NFM or Auto to FM modulation mode.
The main control channel for Weldon Spring is in the middle of the 152 MHz paging band.
If you have a 152 MHz paging site very near you, that can desense the radio and it will not receive the Weldon CC cleanly and knock out your reception. That could explain the 'randomness' of your problem as the paging transmitter turns on/off.
You should also try setting the modulation for that site to FM from NFM.
I just needed to do that for the Weldon site only on my 536HP.
Oddly, I'm able to leave the Imperial site on NFM and it also uses a CC in the paging band just 60 kHz below Weldons.
Why that works is beyond me but it is now needed for Weldon only. That kind of goes against paging signals wreaking havoc with Weldon's CC frequency as NFM mode should help reduce interference from the paging signals in our area and FM mode should make it worse in theory!
Changing Weldon from NFM to FM made mine work perfectly once again even though it had been working in NFM mode just fine a week or two ago.
I don't know what changed but something with the site as I'd made no changes in the scanner whatsoever prior to Weldon Spring flaking out on me.
Can you copy any of the other Moswin sites? Maybe Warrenton or New Hope would be two good ones to try from your area. I get them both here in Chesterfield but they can stay on NFM. They also work well in FM mode but only the Weldon site needed to be in FM mode.
You can switch to FM mode by simply hitting the Channel button while scanning or searching the Weldon site. Once the radio shows it is held on any talkgroup in your Weldon favorite list, press and release the function knob and then quickly press and release the Channel button until the display shows Modulation FM, not NFM or AUTO. You only have about three seconds max between key presses before the Function mode times out!
This wll force Weldon to use FM mode but leave the other sites at NFM.
Once it is on FM, let the Function mode timeout and then press the channel button again to release the channel hold and see if you do not start hearing Weldon's site again.
The same thing can be done globally by editing the Band Defaults and changing the 150.8 line from NFM to FM/ 5.0 kHz. This can be done from Sentinel's edit profile menu or from the settings page of the radios menu.
If you do any of these in the radio, make sure you read the radio into Sentinel and then save it otherwise the next time you use Sentinel, it will revert the settings back to defaults of either Auto or NFM. Of course if FM does not work, then reading and saving from Sentinel is not needed.
If you do the global method of setting FM mode for the VHF range, then the change I mentioned first should be set back to AUTO as that change will overide the global setting.
Actually, if you have other sites searching or scanning with the same quick keys as your Weldon site, you should probably use the global method. Either way should work though.
If you have it set correctly to FM, the display will show FM on the bottom line when a talkgroup goes active. If it still shows NFM, you missed a step or function mode timed out on you!
Another thing I've found for all the VHF Moswin sites in my range is to use Manual P25 mode settings. Set the P25 Threshold Mode to Manual and then set the P25 Threshold Level to 7. Auto mode caused me problems. Manual mode with a level of 7 worked the best.
This needs to be set for each site at the site level from the radios menu system or from within Sentinel.
My 536HP started doing the same exact thing as your 436 is doing maybe about a week ago at the most. And like you, all my other digital systems and sites continued working just fine.
Some of my instructions may need some translating as you have the 436 and I used the 536 for my instructions. My 436 is at work and I don't recall if all the buttons are labelled the same between the two models. I also don't recall if the modulation mode displays on the bottom line on the 436 or another display line.
Please report back here if this helped or not!
edit: one other thing you can do is remove all frequencies from the Weldon site except 152.660 and 154.695. The others are not needed as the scanner will tune them from the control channel data. That eliminates any chance that you are hanging up on a frequency that may have local interference on it. You only need the Primary and Alternate control channels programmed at the site level.