If they are price-fixing it's very simple to catch them. You get a crew together and stake out as many stations as possible, recording the date/time that prices change. A number of them will change prices within minutes of each other; the really greedy ones will do it damned near simultaneously, to the minute.
I don't know what is meant by "co-op" but if it's your area distributor, that's legal. In fact, they're required to notify their dealers of a price change prior to delivery; and it's legal for the affected retailers to immediately change prices prior to the actual delivery. Only exceptions are in states where an official authority sets a cap on profit per gallon and the retailers are under constant scrutiny. My state isn't regulated.
It is also legal/required that the oil companies notify their franchisees/company-owned stations when they increase, as well as legal for all the stations in the chain to change prices simultaneously.
However, let me state again that it is illegal for retailers to get together and price-fix; such as in an "Association of Gasoline Retailers" scenario. That's why you rarely even hear of such organizations any more....the stigma attached.
Within hours after 9/11, a number of retailers in my area jacked up prices to $5.00 per gallon. It was never proved that they communicated amongst themselves, but they were sanctioned for gouging and ordered to reset their prices at pre-9/11 level and fix them there for 6 months.
I'm not saying they don't price-fix in your area, and I can't say with certainty that they aren't doing it in mine. What I do know for a fact is that the retailers are smarter than to be caught doing it as the individuals change prices hours and even a day apart from each other. If you can stake them out and determine they're doing it simultaneously, notify your state Attorney General's office and they will investigate....as well as prosecute if they catch them.