Brian,
Do not have an answer but have similar issue.
Have 1964 American Standard Arcoliner (167,000 BTU/hr)with General Fittings (Rhode Island tankless coil, GPM is not legible) with 30 gal aquaboost storage tank.

Just bought house in Boston suburb.

Unlimited hot water it seems except lower flow than needed in upstairs bath.
Kitchen, bathroom in basement have 2 gpm hot and cold. Shower unknown because of single control valve but good pressure and hot water.
Ktchen sink, 1st floor bath and slop sink in laundry have the same. No issue.
Upstairs bath has approx 1gpm hot and 4 gpm cold at the tub filler. Sink has approx 1gpm hot and 2 cold.
In order to shower, hot must be on full with 1/10 turn of cold.Two separate valves. This is after I choke the cold at shutoff in basement so that faucet flow at sink is severely reduced when toilet is flushed. Back flushed with the dime in the sink faucet with no increase in flow to tub. Barely usable as long a no one else turns on hot water in house. Tub fills real slow with hot and shower head is adjustable water massage. It works great in basement. Supply line into tankless looks like 3/4' with 1/2" out.
Cold water run to bathroom is same size. I understand lower flow at the sinks because of the reduced connections by why the low flow to the tub. Entire basement has finished plaster ceiling so I am trying "black box" box approach before opening up. Is it possible that the tankless coil is clogged. Quote to replace is $1300. When I removed the faucet stem get good initial pressure but presure/flow drops off immediately. Tankless mixer set at 140 deg with circulator controlled by thermostat in storage tank set at 120 deg
Any feedback would be helpful
Thanks
FMH