A simple thing to check are the fuel caps. If only one of the filler caps is vented and on the wrong wing you can have a similar problem to that which your experiencing. If only one filler cap is vented (it is common to have both caps vented nowadays) the vented one should be on the starboard wing. The port wing is always vented of course because of having the vent pipe attached. This problem can be accentuated when both tanks are full to the brim and the overhead crossover pipe is also full of fuel.
Its worth flying with one tank only selected to see if the problem your having still persists or does so on only one of the tanks. It is normal to operate a C172 with both tanks selected. Cessna warn that an air lock is possible, although at high altitude, at the point both tank feed pipes are joined. They therefore recommend that both tanks are not selected at high altitudes. Your operating at low altitudes of course but ..........