The problem you are having is a color convergence issue, and not a terrible one at that. From the intermittent nature of the problem I can almost guarantee you have a bad solder connection on the IC's (chips) responsible for the color correction. It can be a DIY repair IF you have soldering experience and know the safety protocols of electronics repair.
There are some circuits that align the RED, GREEN, and, BLUE, picture tubes to make the picture you see.
**Please understand that by opening a TV you are exposing yourself to many dangers, including electrocution. Be sure that the set is un-plugged and keep one hand behind your back whenever possible to avoid a shock through the heart as this CAN KILL YOU. If you have zero electronic experience I would not attempt this repair.** OK, public service announcement over.
The components that do that job are called "convergence" circuitry. The usual components to fail are the "Convergence Amplifiers" which are usually a pair of Integrated Circuits mounted to a large heatsink. (STK-392) may be printed on the IC along with some number such as 110, 160, 120.
Sounds like you may have a bad solder connection. The fact that it does not remain broken tells me it is not very serious. But I will give you a quick primer on how it's done, if you want to attempt it then I can claim no responsibility if you get hurt. Many MANY problems on these older TV sets are caused by heating and cooling of the components over and over through the years, causing tiny cracks in the connection to the PCB. Dissimilar metals contract and expand differently, Lead tends to get cracks in it around the Hot components (anything mounted to a heatsink is prone to this problem). Some sets will get better as they warm up, some sets will get worse. Sometimes banging on the TV can bring it back momentarily, not a suggested fix though ;)You can get yourself a soldering iron from the local electronics for under $15.Solder is pretty cheap, any 60/40 Lead solder seeems to flow the best for me.Also get youself some "solder wick" to help remove the old solder.Do youself a favor and solder up everything mounted to a heatsink. I would spend 30 or so minutes touching up any "problem areas" (hot stuff).-Concentrate on areas that look like they operate hot. And LOOK CAREFULLY, swollen capacitors and cracked solder joints are easy to miss, and your problem will persist if you miss the bad solder joint.Getting a magnifying glass and a bright light will aid you a BUNCH!Flip the main "chassis" over and you have access to everything, it takes some un-dressing of the wires to get the chassis to come free, take pictures if it helps you remember the layout. PLEASE BE CAREFUL.
You can look up these parts online easily (the convergence amplifiers) with the exception of the burnt resistors and fuses which you usually need a service manual(which you can also purchase online). The STK's price will vary from $15-$90 depending on which one, and the resistors have to be of the metal oxide type so as not to change value with heat.
One place to get parts is www.partstore.com
Service manuals here www.manualsparadise.com
If you want a professional to repair it, be prepared to part with around $200 if it is a bad solder joint, and $300 if it is bad IC's. Sometimes we elect to replace the IC's as they could be damaged by the voltage coming and going repeatedly.
If your problem persists after soldering then look around for swollen capacitors on the circuit board. They can swell up when bad, or leak a fluid from the bottom. That too can cause severe problems and is a common issue.If you STILL have a problem then you may need a service manual, or a service technician. This problem could be fixed for under $200 by a pro, save yourself a headache and a possible shock, call in a qualified tech. Hope that helps! Mike