Do a compression check as soon as possible it is rather easy to do! Could be leaking compression at warmer temps and not at idling speeds or cold. I am in the middle of deciding how I am going to approach a motor replacement because of a blown head gasket that torched a channel in both the head and the block between cylinders 3 and 5. Ran good the weekend before and like crap the next weekend wifey claims that we were not getting the speed out of it towards the end of the season last year before this happened I didn't notice it myself because I only run it WOT for short periods of time when returning home, but I now believe her.