2003 2500 p0300 Code

[Duffy302]

My GMC 2500 with 120000 miles has the engine light come on around 70MPH after motor has warmed up. Goes on and off at random in that speed range.
Code says random multiple misfires. Has new plugs and wires. Local shop says it sounds like intake gasket. but said it would be expensive to fix. looking for any advice . Thanks