What Causes Randomly Occurring Weakness In Chest And Legs?

two weeks ago had extreme weakness in my chest and arms but no pain. went away. 5 days later rapid heart beat for about 15-20 minutes, went away then weakness in chest and arms so went to ER. everything checked out fine. had EKG, blood work, etc. I'm scheduled for eckocarido.... in 2 days. everyday off and on I get weakness in chest and 2 days in legs. then by end of day I feel like nothing has happened. No pain just weakness. now today I had cold sensations and chills, they're gone now. one minute i'm so weak and the next hour or so I feel like I could run a marathon. Very annoying. getting ready to travel in 2 weeks and concerned. Could this possibly be a virus that need to run it's course? My doctor said they are just eliminating things as of now.