When I try to print from the DOS version of InStat or InPlot, I get an incorrect message saying that the printer isn't ready, or the printer just hangs.

In the command line that launches the program, add "/R", like this:

C:\INSTAT2\INSTAT.EXE /R"

That prevents the program from trying to detect whether the printer is ready, a process that doesn't work with networked printers, printers connected through switch boxes, and probably some other situations.