Scope Zero query

[VALE BOY]

I have a Richter 4-16x50 IR with side wheel range adjustment and recently it's proving to be inconsistent in it's shot placement. It's atop a HW98 .177 which until recently was a real tack driver
(and great at long range too hitting the bucket over 60 times out of about 70 shots at the Boinger Bash)

The problem is that after a couple of shots impacting approximately on the point of aim, at 20 yards it then shoots low about 6inches then the next shot maybe in the vicinity of the first 2 then it shoots high an inch or so then the next one can be about 4 inches low. The discrepancy is all on the vertical (elevation) plane as there is little or no differential in the horizontal (windage) plane. The scope and stock screws are all tight and I've tried 3 different pellets JSBs Weihrauch and Air Arms which all were on target previously.
All were shot from a bench rest where previously I could obtain 5p groups at 30 yards.

Can it be due to some internal fault with the reticle and if so are these scopes fixable and if so who can carry it out??

There is no obvious surface damage to the scope or rifle but it may have fallen over in the bag at some point and focuses fine and the clarity is good.

Hoping that some of you may have some suggestions to a solution.
Atb
Graeme