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Being that this problem comes and goes and based on the inputs you have provided here I am thinking fuel contamination, we have seen a lot of issues in certain area's of the country lately where the ethanol was mixed too heavy or water contamination in the tanks that causes issues like yours. This can be frustrating because it can be difficult to figure out for some mechanics and is commonly overlooked. Ultimate the solution here if that be the case would be fuel tank removal, draining, cleaning.
On a 1994 model the main concern is running the engine too lean and altering combustion chamber temperatures, 268,000 miles on this engine is quite a feat so this engine is already pretty worn, as long as the check engine light is not coming on and flashing then there is no major damage being done.
Another thought that came to mind for me earlier stood out as a timing alteration but your fuel statements tell me to have you check the tank first. If you need to wait until the weekend then just be sure to monitor the condition carefully, if this only happens full then consider only filling the tank half way this week.