737NG Altitude Hold Disengages

I don't know if anyone else has this problem, but I cannot use the altitude hold when flying online with VATSIM. I use SB3 and have fully programmed my flight plan into the FMC.As I'm being vectored, I use the MCP to dial-in headings and altitudes. However (for example I set 5000' and 250K), the aircraft begins the climb, and about 10 seconds later the altitude hold light extinguishes, and my airspeed setting jumps to 270K.Also, I cannot set the vertical speed. (I have LNAV and VNAV off)However if I select VNAV, it flys properly.This just started after I installed SB3.Thanks in advance to anyone with some insight!Gerry