Hi folks.
I have a plumbed Andreja Premium that I purchased around 3.5 years ago. It's got a PID, that I set to about 250f and I also have Eric's group head thermometer. Idle temp at the grouphead is around 215-220.

Up until recently, my normal routine would be to flush to around 185 degrees, load up the portafilter, lock it in, steam my milk, and about halfway through, pull the shot. Normally, the grouphead thermometer would shoot up to around 210, then quickly come down to around 205, then stabilize at around 200 at around 10 seconds or so into the shot. Now what's happening, is instead of stabilizing, it just continues to drop way past 195.

I tried searching here for some similar posts, but clearly I'm using the wrong keywords, or this is only happening to me. Anyone have any ideas? Any help will be greatly appreciated.

Thanks!

A little bit of scale will reduce heat transfer from the boiler to the HX under dynamic conditions.

Well, I've never descaled it. I guess it's time to. Thanks!

If your brew pressure is lower than normal, there is likely more water escaping via the OPV and heading back to the reservoir. This will take those temperatures down pretty fast.

Check for scale using this: Checking an E61 Espresso Machine for Scale

Thanks Eric,

The brew pressure seems normal, however, I'm noticing a lot more output from the drain chamber. Also, it's been a couple of months since I've done a backflush. I just moved and I can't find my blank portafilter. Hopefully this is just punishment for bad maintenance and I can redeem myself.

. . . and I can redeem myself.

I would suggest you order the following parts from Chris' Coffee:

opv insert
o-ring for opv adjuster
dispersion screen
grouphead gasket
blank basket
Cafiza or equivalent coffee detergent

You and Andreja will be closer friends as a result

Will do! Thanks for the great advice!