Help with SB 401

[Dan Pividal N4VET]

I have an SB 401 transmitter with a problem.
When I tune it up I show 250 MA of plate and .2MA of grid at full output.
However, while in tune mode the grid current roles off to 0 in about two to
three seconds.
The plate stays at 250MA and output stays steady.
When this happens if I wait about one minute and go to tune again the grid
current is back up but begins to role off again.
At times power output is about 10 watts out and I have to turn level control
full clockwise and then grid current pops up and power output goes to full
output of about 90 watts on 28.400.
This problem seems to happen more if the unit has not been used for a time.
Once I start to transmit for a few days the problem happens less often.
Any help as to where to start to look would be great.
Thanks, Dan N4VET

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