Unstable ISTD only in no-gas Mode

The ISTD of my 7700 is fluctuating immense but only in no-gas mode, in He-mode is everything fine with the stability-graph. I have a recovery from about 20 to 180% with all 3 ISTD’s (Sc,In,Lu), they have the same deviation in the same way. It happens spontaneously with every kind of sample, even with a blank.  I’d be happy if someone has any suggestions to solve this problem.

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