1993 Plymouth Voyager rough idle, gas smell

Engine Mechanical problem
1993 Plymouth Voyager 6 cyl 244000 miles

My 93 voyager has had an intermittent problem these last three weeks. It starts no problem but idles really rough for a couple minutes. Almost like a lawnmower, shaking and running really rough and then churning into an almost normal pattern and ok. I noticed the first couple times the weather was damp, it had rained overnight. I haven't used it much since this problem but had to move it twice this week and noticed a faint but obvious gas smell when I got out. That is scaring me. Suggestions? Also guesstimate of cost to repair?