I have a Polar FT1 - the cheapest one - but some of their more expensive models (e.g.
RS300X) have Interval Training features that might do what you want. Interval Training means you have a program to make sure you do e.g. 5 minutes at ~80% of your max heart rate, then 10 minutes at ~60%, or whatever, and it will beep at you when you need to speed up or slow down.
edit: Timex also do HR monitors with interval training, and are a bit less expensive e.g. their Race Trainer range.