IMHO, with that many plant sites and that small of a res. its gonna happen.

also used GH 3-2-1, 1-2-3 for quite a few grows initially in a 3" tube system with small res. 18 gal/28 sites.

went to buckets with 6" netpots and 20gal/6 plants still had the problem you have,just not as bad

SO, checkout Latewoods legacy a sticky post. it uses a whole different GH reciepe, and it works much better
the Ph runs more like it should. stable until nearing a nuit. change,then it starts changing about 0.1-0.2 units daily

Do you make up with Ph water or a nuit solution?

Read Latewoods legacy