Older Beeman R7

Now have a Hawke 2-7 scope securely mounted.

Went to the range today and sighted it in at 25 yards. Was shooting with the forearm rested atop sandbags about in the middle and I kept my front hand off of it. The buttstock was supported at the toe of the buttstock. The trigger on this rifle is decent. I was trying not to grip the rifle too firmly and made an effort to 'follow through' as best I could.

Got the rifle quickly sighted in and the scope adjustments seemed to track well. But soon noticed a lot of vertical stringing and the POI seemed to shift vertically. We're talking three inch differences vertically from shot to shot with no pattern to the frequency. Sometimes I'd end up with two or three shots nearly touching..and two or three more shots up and down from that. Left-Right never varied a 1/2" during the session.

Where is the recommended position on the forearm atop a rest? Middle? Or closer to the triggerguard?

Any other suggestions?

In these rifles do they tend to like lighter or heavier pellets?