Are you sure there's not something other than SQL on the box using a reasonable amount of memory?

If you've got 19GB and the top cache object is 121MB (I'm assuming the total cache size isn't huge?), yet you have memory problems, then chances are there's something else causing issues on the box?

Antivirus installed? Running frequent scans for example?

What sort of problems were you experiencing in the first place?

If it's just this alert, you can reconfigure the alert to a slightly higher threshold or you can probably ignore it in this case.

It is a "relatively" large object, but if it's being queried often, then it's in there correctly.