Hi Hondo,
By any chance, are you using a synthetic oil in the gearbox? If so, that often causes popping out of gear just as you describe. A switch back to standard mineral-based 20W50 racing oil (non-detergent) in a 4 speed box will usually cure the problem. In an OD gearbox, use 30 to 40W racing oil instead.
Even if you are using mineral-based oil already, when is the last time it was changed? I'd try fresh oil in there, before anything else. Might be all that's needed.
If it's not the oil, i.e. you are already using non-synthetic oil and/or fresh oil doesn't help, another common cause is the shifter rails and detents in the top cover of the box. Here it's mostly an adjustment to compensate for wear, or possibly some gradually weakening/softening springs and/or worn detents that need replacement. This is an opportunity perhaps to replace some of the seals up there, if needed.
These adjustments are detailed in the factory service manuals, but I suspect they are more complicated than they appear and I've heard of more than a few do-it-yourselfers who got into problems here. Others were happy with the results. Personally, I've not rebuilt/adjusted a top cover and would send one of mine to one of the experts, rather than mess it up with my own ignorance and lack of experience!
There is a possibility the problem is the result of wear and tear in the box's gears, bushings and such, too, needing more extensive work. But I think this is a bit less likely than the above two causes, which are less costly to correct and are certainly worth checking out first.
John Esposito is "the man" on the East Coast to check out the box and get it back into top shape for you, especially if you want to keep your original gearbox. He and a few others are also capable of uprating the gearbox in some ways, if you are interested in doing so or your car's state of tune demands it. "Uprating" can include replacing brass bushings with steel or needle bearings, increasing the amount of bearing surfaces in some key areas, replacing solid thrust bearings with roller-type, improved hardening on shafts that are prone to wear, etc.
Uprating is largely done for two reasons: 1. To improve the gearbox's durability, expecially with higher output engines that work it harder, by strengthening areas that experience has shown to be weaker and more prone to breaking. And, 2. To reduce drag on HP - some of which is inevitable in any gearbox - which will in turn transmit more of the power the engine is producing to the diff and, ultimately, the rear wheels.
There are also usually rebuilt/exchange g'boxes available through the major vendors like TRF, VB and Moss (Rimmer Bros. or Moss Europe in the UK). Exchange units typically give you fewer choices and are usually only in standard build, with little or no uprating done.
I think TRF does their own rebuilds, at least on the more commonly available gearboxes. I believe they also would rebuild your specific gearbox, if you wanted the orginality and request it (email John Swauger, if you wish).
VB and Moss probably source their gearboxes from rebuilders in England. Getting a rebuilt/exhannge box, you can't be entirely sure you'll get an exact replacement. It might be a "saloon" car gearbox case, with TR6 innards, for example. Should work fine, but there might be some minor cosmetic differences.
Hope this helps and it would be great if this is something relatively simple and inexpensive to fix!
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