'97 Onan 6500 backfire

[Turbofans]

I have an Onan 6500 genset in our Class A. When I purchased the RV a year ago, the genset only had 25hrs of use and is 17years old (RV info should be in my Signature line).
Since then I have put twice that amount on it and am up to 54hrs of use, and from learning on these forums, I understand that it is not a good thing to have a genset sit unused for a long period of time.
Problem is, it starts ok and runs fine, but after approx. 10min of use, it backfires and dies. Restarting usually takes a couple of tries but seems to run ok once restarted. I replaced the spark plugs hoping this would resolve the issue. It starts much easier and runs much smoother than before since replacement, but the backfire issue is still present.
Any ideas? thanks for

[BFlinn181]

Backfire is usually a timing issue. Spark plugs didn't hurt, but a full tune up is probably needed.

[kentouw]

Check intake gasket. spray some WD40 on it and if it picks up rpms the gasket is sucking air. just a thought from a back yard mechanic .