Truck won't start code P1661

[kurt]

97 with 320k miles. It cranks but won't start. Changed the CPS as it showed that code also but it still won't start. After changing the CPS and clearing the codes, it takes about 3-4 times of extended cranking to come up with the P1661 code.

Today I gave it a sniff of ether and it only ran on that. Checked codes and got P1661 again. Checked all fuses and they were good. Cycled glow plugs and tried again and nothing. Same code. Tried ether again and ran as long as it lasted. Cleared codes and cranked again and nothing. Same code again and cleared. Tried a glowplug cycle again and it started and ran. Waited a while and shut it off and it started right up. Any ideas?

It happened again today. Started this am when cold ran for an hour and I shut it off. About an hour later it cranked and wouldn't start. Cycled and cranked about 4 times then it started. Fuel shut off solenoid?