555 Timer Triggering Too Easily
The 5V bus needs capacitors across it, near the regulator, near the TSOP4838, and another one near the 555.  Without it, there is ripple on the bus.  The TSOP4838 will false trigger with more than 100mV ripple.  This false trigger will trigger the 555.  Switching the load on the 3V bus will cause sag in the 9V bus, which will be passed to the 5V bus via the regulator, before it can compensate.  You should prove this by measuring the ripple before you add capacitors.