My 78 year old mom's 20 year old (80 in people years) Cavalier (1984 2.2L) suddenly started idling rough but only when in gear. Has just turned over 40,000 original miles!!!! Water pump and alternator replaced past 6 months but had been running fine.

Aside from choking on $2.00 gas prices, if she runs fine in park, and really rough when in drive or reverse (AC on and off) what should I look at first?

Thanks