If adding delays 'solves' the problem - that suggests that the Windows side is not processing the data fast enough - when you read from the serial buffer, you need to read as much as you can get then have you VB code pull it apart. Assuming 9600 baud, that's about 1char per 1mS - thus your message is 4mS long. If you grab the 4 characters,decode then update a graph or write to a file, then 4mS doesn't go too far! Try moving some windows and notice how the problem gets worse! Add network traffic,different speed machines etc and you get a very unreliable system. To prove that the data is backlogged, run the system for a few minutes to allow the backlog to happen, then stop the 8051 from sending - measure how long the Windows code still thinks there's data coming in - if it stops immediately - then there's no backlog.

A simple solution is to have the VB side send a character to tell the 8051 to send it's data back - a polled system. doing this, I would expect the reliability would be a lot better and much less system dependant. Add some data packetising method and the system will be significantly more robust. Welcome to the world of communications!