advantest r3131a TG issues

[kc8gpd]

i scored a advantest r3131a. well i'm having tracking generator issues. the TG starts out strong then after a few minutes the output goes weak and eventually down to nothing. i let the analyzer warm up first, so it's not a warm up issue. i turn off TG (not the spec an) for a few minutes and it comes back strong then weakens over a few minutes again. could it be a power supply issue? anybody have pics / procedure of where i should be looking and what i should replace?

Note: i did use the search function first and found nothing on point that i could see.