The latest piece of equipment to falter aboard Mir, commander Vasily Tsibliyev's stressed-out ticker, has thrown a hitch into Friday's planned spacewalk to restore power from the space station's damaged Spektr module. The 43-year-old cosmonaut felt irregularities in his heartbeat during a ride on a stationary bike and again while trying to sleep Friday night. The tentative diagnosis, reports TIME's Dick Thompson, is stress-induced palpitations. "This guy's been through it all," says Thompson. "He's been in command on Mir for the fire, the crash, all the problems they've had since February." Although an official diagnosis has not been issued, the palpitations do not seem an immediate threat to Tsibliyev's health. Russian officials, who badly need Tsibliyev and Alexander Lazutkin to complete the four-hour "internal" space walk in the lab, are already grousing that Tsibliyev's problems might not be entirely physical. "We have been hearing his complaints of a workload being too heavy since the very first day of his flight," an exasperated deputy mission chief Viktor Blagov told The Associated Press. "Maybe it's just personal and he can't work as fast as we urge him to." Or as fast as the beleaguered U.S.-Russian partnership needs it. Congressional pressure is mounting to cancel further American visits to the aging station. Sending American astronaut Michael Foale in Tsibiyev's place is not an option, not only because Foale doesn't have enough experience in spacewalks, but because the U.S. doesn't want to send him into a possibly contaminated spacelab. "Congress is upset enough with the Mir program as it is," Thompson says. "It's better for everyone, especially the Russians, that Foale remain in the Soyuz, out of danger." With Mir's future riding on it, bet on the frazzled Tsibliyev receiving a clean bill of health from Russian doctors on the ground.