Probably insufficient developer capacity in the more dilute solution. Suggest changing developers, D-76 / XTOL / DD-X etc. should work fine.
Tom
The risk is insufficient developer if diluted.
The machine is continuous agitation, 75 degrees F, 420mL of solution. If we figure that Fomapan processing times would be something like 75% of comparable films, continuous agitation times would be something like 85% of regular agitation, and 75 degree processing would take something like 75% of the processing time for 68 degrees we are down to about half the processing time compared to more standard conditions. If we follow recommendations to have at least three and one half minutes then that implies that one must use a very dilute solution.
I am guessing the unofficial dilution H would be about right, i.e. dilution of ~63:1. However, this dilution would give about 6.7 mL of developer in 420 mL of solution, which is pretty close to the minimum of 6mL recommended for one roll of 35mm film. By comparison, the 420 mL tank on a Photo-therm normally holds four rolls. It would be nice not to be limited to processing one roll at a time, but it would not be out of the question. However, if the developer could be restrained, then I could use more mL of developer, opening up the possibility of doing more than one roll at a time.
I do have a densitometer, so I could do some testing of different developer concentrations and development times. It is possible that 6.7mL would end up being too concentrated to meet the minimum development time, in which case things are even less favorable.
By the way, thanks for the other suggestions (rodinal, T-Max developer, D-76, etc.). I do have bottles of those developers, so they are fall-back strategies, but for various reasons I would like to work out a scheme using HC110 if possible.