1999 Volvo S70 ETS light and rough timing during rain or fo

Have you had Recall 155 done already on your car? If not it could correct the problem you are having. If it already has being done then you need to have your vehicle properly diagnosed. You could have a failing electronic throttle module. The module was covered under the extended warranty 10year/200k miles but looks like you went over on miles. If the throttle is bad you can try and have Volvo goodwill the throttle?
They may be able to rebuild the throttle for you. If they can you may need to have software downloaded but you willneed to go to Volvo or a Volvo independent shop for that.