Zenith that goes PFFFT!

[Marantz7t15]

We have a zenith we bought new in 1986. It has run at least 12 hours a day for the last 17 years. Starting a few years ago, occasionally upon start up in the morning, the console will run for about 30 seconds and then PFFFT! The screen goes dead, but you can still hear static. Repeated start and PFFFT! cycles eventually lead to it staying on, and then it plays for hours! Half the time when you hit the power button, it simply comes on. What would cause this? Am I going shopping for a new set?