[Heathkit] V7A VTVM unstable resistance reading?

[Stephen & Marilyn Haines]

The voltage readings for AC and DC are okay. All ohm range readings are 
"shaky" -- that is, from the correct reading, the needle bounces wildly 
to the right at perhaps a frequency of 2 or 3 times a second. Sometimes 
the meter will settle down for 5 seconds and read okay. The tubes are 
very old, with no way to test them. Any ideas, please?  Steve, W9CPI/ZP9EH