[Amps] Slow Keying Issue

[nx5m at txcyber.com]

I recently got an Ameritron AL-1200 that I do not use often so I never
noticed this issue until this past weekend during the CQ 160 CW contest.
I wanted to see how the amp played on 160m and I might add that it works
well.
I have it paired up with a Yaesu FT-990 and using the REMOTE jack from the
radio to key the amp using VOX.
During the contest, many times when I called another station, they did not
get the first part of my call (NX5M).  What they would get was X5M or 5M. 
At first I thought it might be because of someone switching between their
receive antennas.  A bit later I started watching the wattmeter and
noticed that the power was only about 60-70w when I first started sending.
 Although the relay was closing instantly the power output of the amp was
not really kicking in until after the first or second letter of my call
was sent.  That certainly would explain why some were not getting the
first letter or two of my call...especially when the person could not hear
a 60-70w signal.  I tried everything I could to resolve this situation
with no success.
Finally, I just connected a footswitch and once I did that all was well.

Does anyone have a solution to this problem?  I do not like using a
footswitch so if there is a way to make this work I'd certainly appreciate
getting a solution.

73,
Bob NX5M