I've had my riviera for 3 years now, and it just started acting up last night. It turned over ok, but would not start until I lightly pressed the gas pedal. when it got running, the idle sounded a little different than normal- I can't quite put my finger on it, but it was different. I went out to start it this morning, and it started fine, so this leaves me clueless. Any ideas?

Are you getting any trouble codes?
Are you able to verify fuel pressure once you put the key to On, and then a few minutes afterwards?

Some good things to do until you can check for codes, and the fuel pressure would be to get some MAF cleaner and clean the MAF'* thermistors, also be sure the tip of the MAF is not coming off and spinning around in the throttle body bore, it does not happen a lot, but it happened to me a while back, and I had to push the pedal a few times to get it to start.

Before suggesting anything else we need to know what the fuel pressure is.