This is without a doubt a faulty capacitor situation as I can speak from experience in working on Motorola sets. You will notice that the capacitors used throughout those modules are a translucent gold color, typically with the name "ERO" on them (which was a German manufacturer). Motorola was one of the very first companies to start using foreign parts in domestically produced consumer products starting back in the early 60's. Intermittent faults with Motorola TV's such as the TS-915 chassis and others from the late 60's-early 70's are attributed to those capacitors which typically have terrible dielectric leakage. Those caps were basically junk when they were new. Rather ironic that the Germans put out something of inferior quality as they are usually known for engineering overkill.
A word of advice; use good quality replacements! I have always used Sprague "Orange Drops" for tube sets, but for the Motorola's and other low voltage solid state applications, I particularly like the DME series from Cornell Dubilier because of their high quality and smaller size (comparable to the originals) so that they neatly fit on the PC board without "over crowding" if you will. I have used these many times on Motorola sets with great results. I strongly advise that you replace ALL of those gold "ERO" caps on all of the modules. Those leaky caps are loading down other important components that you don't want to have fail... They will cause all sorts of intermittent problems, from washed out video, weak color, to the deflection problems such that you've already experienced. If Motorola used a better cap on their solid state sets, I think it would have greatly improved their reputation.
Since we're on the topic of a TS-915 recap, its also very important that you replace the 3 or 4 electrolytics on the convergence board. Those caps are clustered near the compactron socket that is used to connect the convergence yoke. I've had those lytics fail on me before, as in a direct short out! It took out the vertical oscillator transistor which also happens to be on the convergence panel in those sets. Any combo lytics in cans such as the main filters are going to be fine. Just replace those ones on the convergence board and you'll be all set.
I attached a link (below) to the datasheet from Mouser electronics for the DME caps. They are available from 100-1000VDC, just order the type per the original cap. Also note, most if not all the original ERO caps are going to have their values listed in nanofarads (nF) which is typical of the European caps. Nanofarads are 1 billionth of a farad. Since most caps sold today are marked in microfarads (uF), which is 1 millionth of a farad, you will need to convert the "nF" value to "uF" simply by diving the nF value by 1000. It seems to come up often, so I figure I'd mention it.
Once you take care of the re-cap, a TS-915 will have an excellent picture. They are great sets once properly brought up to par.
http://www.cde.com/catalogs/DME.pdf