S13 Redtop Lagging issue

[NL-S13]

Wondering if anyone could help me pin point the issue. When I'm not under boost for a period of time( driving in slow traffic and can't get much boost built) the car tends to lag and not build up Set boost level when I hammer on it after traffic etc. Set for 15psi and after driving in traffic or under low boost it will reach 5-9psi Max but if I shut the car down and let it sit for 5-10 minutes and then try and hammer on it the lagging has gone. any suggestions help as this gets fairly annoying. It is a 1991 Silva K

I have also noticed lately that all the packing from the cat is slowing coming out of the end of my exhaust.