SET !VAR1 EVAL("var randomNumber=Math.floor(Math.random()*3600 + 1800); randomNumber;")
wait seconds={{!var1}}

es:

SET !VAR1 EVAL("var randomNumber=Math.floor(Math.random()*10 + 1); randomNumber;")
URL GOTO=http://www.....com
' waits 1 to 10 seconds
WAIT SECONDS={{!VAR1}}