1958 Table-Top Motorola Needs Help

[jstout66]

i was wrong on the tube. the tube I replaced was a 6BQ7. Also changed the 6BE6. All the others tested fine. It's a Motorola table-top, in a blonde cabinet, and even has a tuning eye. The model number is 10T28B The tube line-up is: 6BQ7- RF Amp, 6BA6 AM-FM IF,6BA6 FM IF, 6BE6-AM ConV, 6AU6- FM Lin, 6AL5 FM Ratio, 6AV6- AM Det, 6AQ5-Pwr Amp , 6X4-Rect and finally the EM81 tuning eye tube. The radio has no hum and works great, but after 45 minutes gets so garbled you can't understand anything. Have re-tested all tubes when it does this, and they still test fine.