Others should comment a well because I admit I'm biased. The fact that changing from a 22A to a 16A improved your temps does say the first PSU was having problems despite it's ratings.
You really should have a good quality PSU with > 20A on the 12V rail or a dedicated non-ATX PSU for the heatbed for good ABS temps. The MOSFET change can help, but I think you're still not getting enough power from the PSU. If you're hesitant on getting a better PSU, then trying the MOSFET change and making sure all the wires are overly large and solidly connected can help.
When the heatbed is heating, how hot does your MOSFET get? Remember the hotter a MOSFET is, the higher the resistance (it can easily double or triple). Thats part of the reason the MOSFET change can help, it starts out with much lower resistance, heats slower, still has less resistance even after it warms, and never gets anywhere near as hot.
You really should have a good quality PSU with > 20A on the 12V rail or a dedicated non-ATX PSU for the heatbed for good ABS temps. The MOSFET change can help, but I think you're still not getting enough power from the PSU. If you're hesitant on getting a better PSU, then trying the MOSFET change and making sure all the wires are overly large and solidly connected can help.
When the heatbed is heating, how hot does your MOSFET get? Remember the hotter a MOSFET is, the higher the resistance (it can easily double or triple). Thats part of the reason the MOSFET change can help, it starts out with much lower resistance, heats slower, still has less resistance even after it warms, and never gets anywhere near as hot.